Coolerless photonic integrated circuits (PICs) for WDM transmission networks and PICs operable with a floating signal channel grid changing with temperature but with fixed channel spacing in the floating grid

ABSTRACT

A coolerless photonic integrated circuit (PIC), such as a semiconductor electro-absorption modulator/laser (EML) or a coolerless optical transmitter photonic integrated circuit (TxPIC), may be operated over a wide temperature range at temperatures higher then room temperature without the need for ambient cooling or hermetic packaging. Since there is large scale integration of N optical transmission signal WDM channels on a TxPIC chip, a new DWDM system approach with novel sensing schemes and adaptive algorithms provides intelligent control of the PIC to optimize its performance and to allow optical transmitter and receiver modules in DWDM systems to operate uncooled. Moreover, the wavelength grid of the on-chip channel laser sources may thermally float within a WDM wavelength band where the individual emission wavelengths of the laser sources are not fixed to wavelength peaks along a standardized wavelength grid but rather may move about with changes in ambient temperature. However, control is maintained such that the channel spectral spacing between channels across multiple signal channels, whether such spacing is periodic or aperiodic, between adjacent laser sources in the thermally floating wavelength grid are maintained in a fixed relationship. Means are then provided at an optical receiver to discover and lock onto floating wavelength grid of transmitted WDM signals and thereafter demultiplex the transmitted WDM signals for OE conversion.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. Ser. No. 11/106,875, filed Apr.14, 2005, now U.S. Pat. No. 7,636,522 which claims priority to U.S.provisional application Ser. No. 60/563,161, filed on Apr. 15, 2004,each of which is incorporated herein by reference in their entirety.

This invention was made with Government support under Contract NumberW31P4Q-04-C-R074 awarded by the U.S. Army Aviation and Missile Command.The government has certain rights in the invention.

FIELD OF INVENTION

This invention relates generally to photonic integrated circuits (PICs)and more particularly to uncooled PICs operating as WDM transmitters orreceivers and also particularly such PICs that operate with a floatingwavelength grid of signal channels where the spatial separation betweenthe signal channels is continuously maintained fixed.

DEFINITIONS

In order to better understand the disclosure, the following definitionsare offered relative to certain terminology that is employed throughoutthis disclosure:

The term, “modulated sources”, includes directly modulated lasers and cwlasers with external modulators or any element or elements that providea modulated signal at a given wavelength. The term, “external”, as usedin the art in this context means “independent or separate from” thelaser and the modulator is an integrated device on the same substratewith the laser.

Lasers or laser sources are the same element.

An “element”, which is synonymous with “component”, means any active orpassive optical device integrated on a photonic integrated circuit (PIC)that performs a function on the PIC. Examples include, but not limitedto, a laser, modulator, PCE, MFE, an element with a fixed insertion losswhich can be fixed or set at a bias, a waveguide, a combiner ordecombiner, a coupler, or splitter.

Reference to “WDM” is intended to include “DWDM” and “CWDM”. The term“channels” or “signal channels” has general reference to modulatedsources on the PIC chip, such that if there are N signal channels, thenthere are N modulated sources. Reference in this description tocircuit-integrated laser source/modulator signal channels is nominally alaser source and its associated modulator together comprising amodulated source and providing a modulated signal output. In thiscontext, such a channel is also functions as an optical waveguide.

“Active region” as employed in the description in this application meansthe region in a semiconductor device where carrier recombination occurswhich may be comprised of a single semiconductor active layer ormultiple semiconductor layers with any associated optical confinementlayers, as is well known to those skilled in the art.

A “channel” means an integrated, optical signal channel waveguide pathin a single-channel EML or in a multiple channel PIC that minimallyincludes a modulated source or other active element, such as aphotodetector (PD), for propagation of an optical signal and where, inthe photonic integrated circuit (PIC), there are N signal channelsformed in an array across the PIC where N is two or greater and or in anEML there is N=1.

A “combiner or decombiner” means a wavelength selective combiner ordecombiner and a free space combiner or decombiner. A “wavelengthselective combiner or decombiner” is a wavelength discriminatingcombiner or multiplexer of wavelength channel signals. A “free spacecombiner/decombiner” is a wavelength indiscriminate combiner withrespect to combining different wavelength channel signals. Moreparticularly, the output power, in units of dBs, from a wavelengthselective combiner may be defined as

${{\sum\limits_{i}^{N}P_{i}} - {IL}},$where P_(i) is the optical input power into the combiner, N is thenumber of outputs and IL is the insertion loss factor. For the freespace combiner, it is typically defined as

${{\sum\limits_{i}^{N}{P_{i}/N}} - {IL}},$where P_(i) is the optical input power to the combiner and N is thenumber of inputs and IL is the insertion loss factor. It can readily beseen that the difference power output is the prime difference. Examplesof wavelength selective combiners/decombiners are, but not limited to,an arrayed waveguide grating (AWG), an Echelle grating, a cascadedMach-Zehnder interferometers, a quasi-wavelength selective star coupleror an elliptical supergrating. Examples of free spacecombiners/decombiners are, but not limited to, a multimode interference(MMI) coupler, free space coupler, star coupler or any such opticalcoupler with a multimodal coupled region.

As employed in this description, a photonic integrated circuit (PIC) maybe any semiconductor device, including a silicon device, which has atleast two elements integrated in an optical circuit. Thus, a PIC can bean EML, TxPIC, RxPIC or any other circuit with a plurality of elements,passive or active, formed in the circuit.

“Laser emission wavelength” means emission output wavelength of a laseror lasers.

“Active region wavelength” means the wavelength of the photoluminescencepeak or the gain peak in am active region of element of in a signalchannel formed in a photonic integrated circuit (PIC), such as, forexample, an active region of a laser source, a modulator, a monitoringelement such as a photodetector or an power changing element (PCE) suchas a semiconductor optical amplifier (SOA), a variable opticalattenuator (VOA) or a variable gain/loss element such as a combinationSOA/VOA device.

“Spectral spacing (Δλ)” means the difference in laser emissionwavelengths between adjacent signal channels in a photonic integratedcircuit (PIC) or between discrete modulated sources.

“Laser detuned offset” means the difference between laser emissionwavelength and the laser active region wavelength for a signal channel.

“Positive wavelength detuning” or “positively detuned wavelengths” meansthe laser detuned offset that is greater than zero. As used herein,“positive wavelength detuning” can also include slightly negativewavelength detuning, i.e., just below zero, since in fabricating PICs,it is possible that intended positively detuned wavelengths can end upslightly negatively detuned by a few nanometers.

“Laser-modulator detuning” means the difference between the laseremission wavelength, and the modulator active region wavelength relativeto the same signal channel.

“Operation window” means the range of laser source channel emissionwavelengths over which there is acceptable loss and acceptable bit errorrate (BER) performance of the modulated sources for a particularspecified application of the PIC.

“PCE” means a power changing element (a power varying element or a fixedloss element) integrated in one or more of the channels of a photonicintegrated circuit (PIC) that changes the power level of the lightpropagating through the element. Examples of PCEs are photodetectors,semiconductor optical amplifiers (SOAs), variable optical attenuators(VOAs), or combination SOAs/VOAs which may also be referred to as ZOAs,Δ-β coupler, a Mach-Zehnder interferometer that changes the phase oflight split between the interferometer arms, or the deployment of anabsorption region of a predetermined length formed in the signalchannel.

The wavelength grid of a plurality of modulated sources as well as thewavelength grid of a combiner or decombiner is also referred to as a“wavelength comb”.

A “slew rate” is defined as a rate related to how rapidly the wavelengthgrid moves or changes in frequency in a coolerless (heated) ambient,which can be measure in nm/° C. or GHz/° C. As an example, on a TxPICdescribed herein, the laser sources slew rate is around 16.5 GHz/° C.such as in an ambient between approximately room temperature andapproximately 70/° C.

DESCRIPTION OF THE RELATED ART

For long haul optical telecommunications in the past, the opticaltransmitter has primarily been comprised of optically fiber coupleddiscrete semiconductor laser sources and discrete external modulators.In most cases, the laser source of choice has been the DFB laser and themodulator of choice has been the Mach-Zehnder lithium niobate modulator.More recently, the integration of these two components have come intocommon commercial reality comprising a monolithic DBR or DFBlaser/electro-absorption (EA) modulator (EAM) integrated on the samesubstrate. The laser source of choice in most cases has been the DFBlaser. These devices are also referred to as an EML (electro-absorptionmodulator/laser). It is highly desirable to monolithically integrate anEA modulator with a single-frequency laser, such as a DFB or DBR laser.Such externally modulated laser sources, such as an EA modulator, aremore attractive than direct modulated laser sources because of theirintrinsic low static chirp. These EMLs have the advantage over previousdiscrete laser/modulator devices in that (1) coupling or insertionlosses between the laser and modulator are reduced or negligibleachieving stable and reliable modulation sources, (2) laser chirp dueto, at least in part, of feedback reflection from the laser/modulatorinterface or the modulator facet is reduced, if not negligible, and (3)costs in producing such an integrated device are lower.

EMLs generally employ multiple quantum wells (MQWs) in the device'sactive region. The issue in fabricating these integrated devices,however, is that the MQWs for the modulator section are required to havea wider effective bandgap than the MQWs for the laser section. This canbe difficult if the integrated laser section and the modulator sectionhave the a common active region because the initial belief was that inorder to achieve the necessary bandgap between these sections, therespective active regions of these devices had to be made witheffectively different bandgaps.

Electro-absorption modulator/laser (EML) devices are now being deployedin transmitter systems for optical transmission networks with bit ratesup to 10 Gb/s. These devices are generally an integrated DFB laser andelectro-absorption modulator and provide for improved performance due totheir integration and lower package costs. The improved performance atthe modulator is augmented by the achievement of high extinction ratiosand low chip characteristics. The waveguide cores in EMLs or arrays ofmodulated sources are preferably AlinGaAs multiple quantum wells (MQWs),which we abbreviate to AQ MQWs as opposed to InGaAsP MQWs or PQ MQWs,improve laser performance at elevated temperatures. See, for example,the article of M. R. Gokhale et al. entitled, “Uncooled, 10 Gb/s1310 nmElectroabsorption Modulated Laser”, Optical Fiber CommunicationConference & Exposition (OFC 2003), Post-deadline (PD) paper 42, pp.1-3, Mar. 23-28, 2008. This paper reports a 10 Gb/s1310 nm EML with AQtwin-waveguides that operates uncooled from 0° C. to 85° C. with fairlymaintained average power and modulator extinction ratio over theforegoing temperature range. The deployment of the twin waveguidesrequires additional growth steps and a good and high yield-reproduciblecoupling mechanism between the DFB laser and the EAM. Simpler approachis a single active region/waveguide core for the laser and themodulator, although, as recognized in the art, back reflections from themodulator into the laser can be an issue but can be dealt with. Thus,1550 nm lasers have all been cooled, such as, for example, with athermal electric cooler (TEC) upon which the laser is mounted.

In some current EMLs, the DFB laser grating is designed to have a longergrating period than the wavelength of the active region material gainpeak which is referred to as positive detuning and, in some cases, maybe even slightly negatively detuned.

In the article of Randal A. Salvatore et al. entitled,“Electroabsorption Modulated Laser for Long Transmission Spans”, IEEEJournal of quantum Electronics, Vol. 28(2), pp. 464-476, May 2002,discloses a cooled (to 25° C. or room temperature, for example) 1550 nmrange EMLs with a complex-coupled AQ active region/waveguide. Thus, 1.5μm AQ EMLs are known but are not operated uncooled, i.e., they include athermo-electric cooler (TEC).

What is desired is a 1500 nm range EML that can operate uncooled over awide temperature range above and below room temperature while providingsubstantially uniform power output over such a temperature range.

Reference is now made to U.S. patent application Ser. No. 10/267,331.also Pub. No. US 2003/0095737 A1; Ser. No. 10/267,304, also Pub. No. US2004/0033004 A1; Ser. No. 10/267,330 also Pub. No. US 2003/0095736 A1;Ser. No. 10/267,346, also Pub. No. US 2003/0081878 A1, all filed Oct. 8,2002, owned by the assignee herein and incorporated herein in theirentirety by their reference. These applications disclose the firstphotonic large scale integration (P-LSI)-based photonic integratedcircuits (PICs). The InP-based, optical transmitter photonic integratedcircuit (TxPIC) formed in these chips comprises an array of modulatedsources, which may be an integrated array of direct modulated lasers(DMLs) or may be an integrated array of laser sources withcorresponding, optically coupled, integrated electro-optic modulators(EOMs), such as EAMs. In either case, they include an array of lasersources, for example, DFB lasers or DBR lasers. The respective lasersources operate at different wavelengths which are respectively set towavelengths on a standardized wavelength grid, such as the ITU grid.Thus, each of the modulated signals from each laser source/modulator(also referred to as “modulated sources”, which also is intended toinclude directly modulated lasers in such signal channels) is a signalchannel with a frequency different from other signal channels—allintegrated on a single chip. The channel signals are provided as inputsto an integrated optical combiner which may be a wavelength selectivecombiner or filter such as an arrayed waveguide grating (AWG), anEchelle grating, a cascaded Mach-Zehnder interferometer or aquasi-selective wavelength star coupler. On the other hand, the opticalcombiner may be a power coupler, star coupler or a MMI coupler. Examplesof the foregoing can be seen in the above identified incorporatedapplications, in particular, U.S. application Ser. No. 10/267,331,supra.

The InP-based, optical receiver photonic integrated circuit or RxPICcomprises a semiconductor chip having an input for a multiplexed signal,which signal may be first amplified by an off-chip EDFA or by an on-chipoptical amplifier. The signal may then be demultiplexed by an on-chipdecombiner or filter where the multiple output waveguides from thedecombiner comprise a plurality of different modulated optical signalswith the terminus of each waveguide coupled to a respective on-chip,integrated photodetector, such as PIN photodiode. The photocurrentsignals from the photodetectors are provided to a transimpedanceamplifier (TIA) for conversion of each of the photocurrents to a voltagesignal which is an electrical rendition of the optical signal. Such aTIA may be an integrated part of the RxPIC chip. More details andexamples relating to RxPIC chips is disclosed in U.S. patent applicationSer. No. 10/267,304, supra.

In a conventional dense wavelength division multiplexed (DWDM)communication system available today from telecommunication serviceprovider equipment manufacturers, the generation of a lot of heat iscommonplace and is a major limitation to decreasing the size, power andcost of these system. The use of monolithically integrated photonicdevices, such as EMLs, TxPICs and RxPICs discussed above, whichincorporate multiple functions into a single semiconductor chip, cansignificantly reduce the overall power requirements of an opticaltransmitter module. The large-scale integration of these types ofphotonic integrated circuits (PICs) provides a large increase infunctionality with an associated significant reduction in overall power,weight, size and cost. Although integration has been demonstrated toreduce power consumption, the thermo-electric cooler (TEC) or a Peltiercooler employed to cool these PIC chips can use up four to ten times asmuch power as the chip or chips itself that are being cooled to operateat a designated temperature. This large power consumption via the use ofsuch coolers significantly diminishes the effects of improvements madein device power requirements of such large-scale integration devices.Furthermore, the additional power utilized by the TEC increases therequired heat sink size, weight and cost, often exponentially. Thus,there is a major reason, as well as technical challenge, to remove therequirement for a TEC in such PICs.

The major challenge in realizing an uncooled DWDM optical transmitter iscontrol of the operating wavelength of the multiple on-chip laserdiodes. DWDM implies an accurate control of the transmitter wavelength,whereas changing environmental temperature in a TxPIC, for example,inherently works also to change the wavelength of the on-chip laserdiode transmitters. It is an object of this invention to deploy a newand dramatic DWDM system approach, together with novel sensing schemesand adaptive algorithms to provide intelligent control of PIC chips suchas EMLs, TxPICs or RxPICs in an optical transponder or transceivermodule to optimize its performance and to allow these semiconductors tooperate uncooled on a continuous basis. Control of the high speedperformance parameters, like the transmitter chirp, is also required inorder to insure a satisfactory quality data transmission.

OBJECTS OF THE INVENTION

It is an object of this invention to eliminate or substantially reducethe foregoing discussed problems in this art.

It is a further object of this invention to provide a PIC that requiresno cooling and yet meets required performance criteria and provides forinexpensive packaging since a packaged cooler, such as an expensive TEC(thermo-electric cooler), is not required and the requirements for ahermetically sealed package are substantially relieved, if noteliminated.

It is another object of this invention to provide an array of lasersintegrated in a PIC for operation over an extended relatively hightemperature operating range, as opposed to or compared to roomtemperature, where the minimum temperature of the range is maintained byPIC integrated circuit heaters to maintain the operational wavelengthsof the respective lasers, but at higher operating temperatures, i.e.,above the minimum temperature and within the high temperature operatingrange where the heaters are employed to tune the laser wavelengths to bewithin a predetermined frequency spacing relative adjacent lasers in thearray.

It is another object of this invention to provide a PIC for operationover an extended temperature range, such as, but not limited to, ofabout −20° C. to some less than 100° C., more particularly in a range ofabout 20° C. to about 70° C.

It is another object of this invention to provide an array of integratedarray of lasers, such as an array of EMLS or an array of laser sourcesin a PIC, that are not temperature-controlled so that the comb ofwavelengths comprising the optical outputs of the laser array arepermitted to drift within a temperature operating range. However, thewavelength spacing between adjacent lasers in the array are maintainedat a constant value, i.e., the comb of wavelengths of the laser arrayare locked to a fixed frequency spacing where such a spacing among thearray lasers may be uniform or nonuniform.

It is another object of this invention to provide an adaptive opticalreceiver to identify the floating grid of signal wavelengths upondemultiplexing and identifying the signal channels via the fixed spacingof a comb of transmitted and floating wavelengths, tuning to therespective floating wavelengths representing the signal channels andtracking the floating grid of signal wavelengths while the respectivechannel signals are converted from the optical domain into theelectrical domain at the optical receiver.

Another object of this invention is a feedback system that monitors andlocks the comb of wavelengths of an array of integrated laser sources ona PIC with fixed wavelength spacing between adjacent laser sources whilethe ambient PIC temperature changes over a broad temperature operatingrange so that the operating wavelengths of the individual laser sources,with a fixed grid because of their fixed channel spacing, may changewith temperature over a temperature operating range.

It is another object of this invention to provide wavelength detectorsthat are integrated in a photonic integrated circuit or PIC that areemployed to detect output signal wavelengths from active or passivedevices, such as laser sources in a TxPIC or EMLs or from a wavelengthselective decombiner in a RxPIC.

Other objects will become apparent throughout the remaining descriptionof the invention.

SUMMARY OF THE INVENTION

According to this invention, a PIC, such as an EML, TxPIC and RxPIC, areallowed to be operated uncooled and unheated or heated over a widetemperature range so that expensive packaging, such as requiring ahermetically sealed package and TEC submounts with an accompanyingcooler, can be in many cases can be suppressed, if not eliminated, inuse for their operation in an optical transponder or transceiver modulein an optical transport or communication network, whether for long haul,metro or WAN or LAN. The major challenge to realizing an uncooled WDMtransmitter is control of operating wavelengths, such as in DWDMsystems, as understood today, implies an accurate control of transmitterwavelengths, whereas environmental changes in temperature inherentlyoperate to change the wavelength of the transmitters. This invention isdirected to a new DWDM system approach with novel sensing schemes andadaptive algorithms that provide for intelligent control of the PIC tooptimize its performance and to allow optical transmitter and receiverdevices in DWDM systems to operate uncooled. Control of the high speedperformance parameters, such as transmitter chirp, still continuallyinsures that a satisfactory quality data transmission is realized.

Another important feature of this invention is the provision of anoptical transmitter and an optical receiver for deployment in an opticaltransmission network at a terminal end, as a mid-span opticalregenerator or repeater (OEO regeneration), or as a mid-spanadd/drop/pass-through module where the operating wavelengths of theoptical transmitters are floating, i.e., the operating wavelengths arenot held to approximate a wavelength on a standardized grid but rathercan move higher or lower with increasing or decreasing temperature,respectively, but the wavelength grid of the group of opticaltransmitters remains the same and is held constant, such as at a 50 GHz,100 GHz or 200 GHz spacing. This principal can apply equally as well tocurrent and conventional optical transponders now deployed, such asthose employing discrete lasers and external modulators at the opticaltransmitter or photodetectors at the optical receiver as well asemploying channel EMLs.

Thus, the floating wavelength grid principal of multiple wavelengthsignal channels as disclosed in this application is not only applicableto EMLS or PICs but also equally applicable to present day transpondershaving large cooling systems to maintain the transmission channelwavelengths approximate to a predetermined or established wavelengthgrid, such as the ITU grid. Therefore, this invention is a significantand unheard of departure from previous well established standardsrequiring the wavelength operation of individual optical transmitters ina network channel signal generator must be maintain within a smallnanometer range of designated wavelength positions along a wavelengthgrid such as wavelengths in the C band. To the contrary, the wavelengthpositions of the channel signal generator signal channels of thisinvention are allowed to move up or down with temperature within atemperature range, such as within the C band or into other such bands,such as the L band, as long as the wavelength grid spacing of the signalchannels remains fixed. In this connection, the fixed spacing betweenall signal channels may uniform or identical, or they can be non-uniformwithin a predetermined pattern such as, some signal channels having agreater spacing than other signal channels, or monotonically increasingor decreasing in special relation, again, as long the wavelength spacingbetween adjacent signal channels across the floating wavelength grid oftransmitter wavelengths remains continually and substantially fixed.

Another feature of this invention is a coolerless, semiconductorelectro-absorption modulator/laser (EML) or EMLs, or an array ofmodulated sources integrated in a photonic integrated circuit (PIC),such as an optical transmitter PIC (TxPIC), with each EML comprising anintegrated CW operated laser source and an electro-optic modulator, suchas an electro-absorption modulator (EAM) or a Mach-Zehnder modulator,where an active region, as formed in the EML or PIC for guiding lightgenerated by the cw-operated laser source through a formed waveguidecore, contains the quaternary AlGaInAs (AQ) and the laser source orsources are positively detuned relative to the natural photoluminescence(PL) peak or gain peak of the active region material. As a result, theEML or TxPIC can operate over a wide temperature range without requiredambient cooling a provide a substantially uniform output power and laserthreshold current over a wide temperature range, in particular, a widehigh temperature range, such as from around room temperature to sometemperature below 100° C., such as in the range of about 20° C. to aboutbetween 70° C. to 85° C.

It is another feature of this invention to provide a Group III-V basedPIC (e.g., EML, TxPIC or RxPIC), such as an InP-based PIC with two ormore integrated elements, operated without a PIC cooler and employing aheater to maintain a fixed operating point or set operating conditionfor the PIC. The heater for the PIC is applied to at least one elementon the PIC, such as a laser, modulator, semiconductor optical amplifier(SOA, variable optical attenuator (VOA), wavelength selectivecombiner/decombiner or filter or any other power changing element (PCE)integrated in the PIC.

A further feature of this invention is a PIC which includes anintegrated wavelength detector for detecting an output wavelength fromone or more elements also integrated on the PIC. In this regard, thedetector may be employed as integrated wavelength locker for wavelengthstabilization or wavelength grid stabilization. Thus, this featurecomprises a PIC that includes an integrated wavelength control elementwhich is integrated in the PIC with other integrated PIC elements. Sucha PIC can be operated either coolerless (heated) or with a cooler, suchas a TEC.

Other objects and attainments together with a fuller understanding ofthe invention will become apparent and appreciated by referring to thefollowing description and claims taken in conjunction with theaccompanying drawings in which like numerals indicate like structuralelements and features in various drawings. The drawings are notnecessarily to scale with emphasis placed upon illustrating theprincipals of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings wherein like reference symbols refer to like parts:

FIG. 1 is a plan view of a coolerless PIC comprising a coolerlesselectro-absorption modulator/laser (EML) comprising this invention.

FIG. 2 is side elevation of the EML of FIG. 1.

FIG. 3 is cross-sectional view of the EML taken along the line 3-3 inFIG. 2.

FIG. 4 is a graphic illustration of the average power versus ambienttemperature for a plurality of EMLs showing substantial uniformity inoutput power over a large operating temperature range.

FIG. 5 is a graphic illustration of the average laser threshold powerversus ambient temperature for a plurality of EMLs showing substantialuniformity in threshold current over a large operating temperaturerange.

FIG. 6 is a plan view another embodiment of a coolerless PIC comprisinga coolerless EML shown in FIG. 1 which includes a strip heater for theEML.

FIG. 7 is a plan view of further embodiment of a coolerless PICcomprising a coolerless EML shown in FIG. 1 which includes a powerchanging element (PCE) comprising an integrated semiconductor opticalamplifier (SOA).

FIG. 8 is a plan view of a coolerless PIC comprising an opticaltransmitter photonic integrated circuit (TxPIC) employing the featuresof this invention.

FIG. 9 is a plan view of a coolerless PIC comprising an optical receiverphotonic integrated circuit (RxPIC) employing the features of thisinvention.

FIG. 10 is schematic illustration of a first embodiment of an opticaltransmission network having multi-channel PICs utilizing a floatingwavelength grid comprising this invention.

FIG. 11 is schematic illustration of a second embodiment of an opticaltransmission network having multi-channel PICs utilizing a floatingwavelength grid comprising this invention.

FIG. 12 is schematic illustration of a third embodiment of an opticaltransmission network having multi-channel PICs utilizing a floatingwavelength grid comprising this invention.

FIG. 13 is a schematic illustration of a first embodiment of an on-chipwavelength detector integrated in a PIC.

FIG. 14 is a schematic illustration of a second embodiment of an on-chipwavelength detector integrated in a PIC.

FIG. 15 is a schematic illustration of a third embodiment of an on-chipwavelength detector integrated in a PIC.

FIG. 16 is a schematic illustration of a fourth embodiment of an on-chipwavelength detector integrated in a PIC.

FIG. 16 is a schematic illustration of a fifth embodiment of an on-chipwavelength detector integrated in a PIC.

FIG. 17 is a schematic illustration of a sixth embodiment of an on-chipwavelength detector integrated in a PIC.

FIG. 18 is a schematic illustration of a seventh embodiment of anon-chip wavelength detector integrated in a PIC.

FIG. 19 is a schematic illustration of an eight embodiment of an on-chipwavelength detector integrated in a PIC.

FIG. 20 is a schematic illustration of a ninth embodiment of an on-chipwavelength detector integrated in a PIC.

FIG. 21 is a schematic illustration of a tenth embodiment of an on-chipwavelength detector integrated in a PIC.

FIG. 22 is a schematic illustration of an eleventh embodiment of anon-chip wavelength detector integrated in a PIC.

FIG. 23 is a schematic illustration of a twelfth embodiment of anon-chip wavelength detector integrated in a PIC.

FIG. 24 is a schematic illustration of a thirteenth embodiment of anon-chip wavelength detector integrated in a PIC.

FIG. 25 is a schematic illustration of a fourteenth embodiment of anon-chip wavelength detector integrated in a PIC.

FIG. 26 is a schematic illustration of a fifteenth embodiment of anon-chip wavelength detector integrated in a PIC.

FIG. 27 is a schematic illustration of a sixteenth embodiment of anon-chip wavelength detector integrated in a PIC.

FIG. 28 is a graphic illustration of an example of a wavelength grid orcomb of multiple laser sources such as in a TxPIC.

FIG. 29 is a cross-sectional view of an electro-optic modulator in aPIC, such as an EML or TxPIC, with a heater mounted on the top of theridge waveguide of the electro-optic modulator.

FIG. 30 is a graphic, semi-log illustration of the ratio of the frontphotodetector (FPD) or front PIN photodiode or FPIN photocurrent to therear photodetector (RFD) or rear PIN photodiode or BPIN photocurrentacross a ten signal channel TxPIC as a function of temperature.

FIG. 31 is a graphic, semi-log illustration of ratio of the frontphotodetector (FPD) or front PIN photodiode or FPIN photocurrent to therear photodetector (RFD) or rear PIN photodiode or BPIN photocurrentacross a ten signal channel TxPIC as a function of temperature via alinear fit.

DETAILED DESCRIPTION OF THE INVENTION

Reference is now made to FIGS. 1-3 which are directed to a coolerlessPIC in the form of a coolerless electro-absorption modulator/laser orEML 10 comprising this invention. EML 10 comprises, in monolithic form,an integrated laser source 12 and an electro-absorption modulator (EAM)14. Laser source 12 may be a DFB or DBR laser but a DFB laser ispreferred in the embodiments here. EML 10 is provided with a shallowridge waveguide 36, as seen in FIG. 3. However, the ridge can also be adeep ridge waveguide device, rib-loaded waveguide, or buriedheterostructure waveguide.

It should be noted for the purposes of this invention, the modulator 14may also be a Mach-Zehnder modulator (MZM), an example of which isdisclosed in incorporated patent Ser. No. 10/267,331, supra. A MZM maybe a “pure” MZM, i.e., one not operated near its bandedge, or may be abandedge MZM, i.e., one operated at its bandedge. In the former case, acoolerless operation using a heater for the pure MZM is not required.However, it may be still desirable to operate a pure MZM in an uncooledstate (coolerless environment), such as in a case to control modulatorchirp. In the case of a bandedge MZM, a coolerless operation using aheater is required to maintain its operation within the bandedge sincethe wavelength can vary rapidly with temperature when operating alongthe bandedge so that power will fall and collapse if tight temperaturecontrol is not maintained.

As shown in FIGS. 2 and 3, coolerless EML 10 may be comprised of ann-InP type or semi-insulating (InP:Fe) substrate 16 upon which isepitaxially deposited, such as by MOCVD, an n-InP buffer layer (notshown), an n-InP confinement layer 18, followed by a quaternary (“Q”)grating layer 20, which may be InGaAsP (“PQ”) or AlinGaAs (“AQ”). A DFBgrating 22 is formed in Q grating layer 20 in the region of laser source12, as conventionally known and carried out in the art. The structureshown further includes a n-InP planarization/separation layer 24followed by active region 26 comprising AQ, i.e., AlinGaAs which can bea bulk layer but more preferably is a plurality of strained quantumwells and barriers where there may be, for example, about 4 to 6 suchquantum wells. An optional planarization layer 28 of non-intentionallydoped (NID) InP may be provided and followed with an optical NID layer30 of InP, AlinAsu, InAlGaAs, InAlAsP or InAlGaAsP is grown whichfunctions as a stop etch layer forming the shallow ridge waveguide 36.This is followed by the growth of p-InP confinement layer 32 and contactlayer 34 of p⁺⁺-InGaAs as is known in the art.

While an InP-based regime has been exemplified above, other Group III-Vregimes may be employed including a GaAs-based regime.

Appropriate separate metallizations (not shown) are formed on thesurfaces of contact layer 34 which contacts are electrically isolated bymeans of isolation region 35. Isolation region 35 may be a groove or anion implant, for example, as known in the art. An n-contactmetallization (not shown) is provided on the bottom surface of substrate16.

The waveguide core formed in active region 26 is preferably AlInGaAs orAQ which provides for high temperature operation of DFB laser source 12as well as provides for a wider modulator window for the bandwidth ofpossible laser emission wavelengths, particularly for amulti-channel/multi-wavelength PIC. The laser source 12 is largely,positively detuned, i.e., the grating pitch 22 of DFB laser 12 is chosensuch that the laser operates on the longer wavelength side of the gainpeak or the PL peak of active region 26. This detuning provides forlaser performance to be substantially uniform over a wider temperaturerange, in particular, the laser gain is maintained or actually increasessome with increasing operating or ambient temperature. Laser 12 isfabricated to operate at a positive detuned wavelength, for example, inthe range of about 60 nm to about 100 nm from the gain peak. The laserdetuned emission wavelength is close to the absorption edge of themodulator AQ active waveguide core thereby insuring optimal wavelengthcompatibility between laser 12 and EAM 14 without significantlydegrading the performance of the laser source due to the application ofpositive detuning. In other words, the laser-modulator detuning relativeto the emission wavelength of laser 12 with respect to the transmissionwavelength of EAM 14 is red-shifted. A wide gain spectrum in theoperation of laser source 12 is achieved due to the employment ofstrained multiple quantum well layers in laser active region 26. Thisdetuning of DFB laser 12 plus the deployment of the negative chirpregime at EAM 14 provides for initial modulated pulse compressionpermitting the extended transmission of the optical signal over highdispersion fibers thereby resulting in lower BER over comparativelengths of such fibers. The net result is that laser power output andlaser threshold current does not change much over a fairly widetemperature range as illustrated, respectively, in the graphs of FIGS. 4and 5. The graphic data in FIGS. 4 and 5 is the average results for anumber of EML PICS, in particular, sixty such devices. It can be seenfrom the results relative to curve 38 in FIG. 4 that the output power ofthese devices varied between 14 mW and 15 mW, i.e., within about 1 mWover a wide operating temperature range from 15° C. to 40° C. By thesame token, as seen in FIG. 5, relative to curve 39, threshold currentover this temperature range varied only about 5 mA. Thus, it can be seenthat substantially uniform power and laser threshold current is achievedover about a 25° C. temperature range without the application of anyambient cooling when EML 10 is fabricated with AQ active regions andwhere the laser emission wavelength is positively detuned relative tothe active region wavelength and the laser emission wavelength is withinthe wider-provided modulator operation window.

Also, a further benefit that is achieved is that as the detuning of theEAM 14 and the lasing wavelength (the laser-modulator detuning) isreduced, the frequency chirp characteristics are improved resulting in alower BER.

Further, reduced positive detuning of laser source 12 will shift towardthe PL peak and the PL peak will also shift toward the detuned laseroperating wavelength with an increase in device temperature which isbeneficial since the laser gain will correspondingly increase as theambient temperature increases. Thus, an increase in ambient temperatureof EML PIC 10 results in a bandgap shift of active region 26 of DFBlaser 12, reducing the detuning of the gain peak toward the lasing oroperating peak of laser 12, resulting in higher gain. As previouslyindicated, the net result of power output and laser threshold currentremains little changed over a wide operating temperature range as seenfrom the results in FIGS. 4 and 5. As the ambient temperature of EML 10increases, the gain of the laser will move toward the PL peak, e.g.,typically at a rate of about 0.16 nm/° C., as well as the PL spectrumwill move toward the positively detuned laser emission wavelength, e.g.,typically at a rate of about 0.5 nm/° C., with a net result ofincreasing laser gain with increasing laser temperature. Thus, as EML 10heats up, the laser operating gain increases toward the photoluminescent(PL) peak whereas the net effect of most other lasers not having theattributes set forth in this application is that laser gain willdecrease with increasing temperature. Such lasers are usually tuned tothe photoluminescent peak or negatively detuned so that with increasingtemperature, laser gain will fall off at higher temperatures. Thus,positive detuning is an important aspect of this invention in providingfor laser/modulator wavelength compatibility as well as capable ofincreasing gain or at least stabilizing gain as the EML PIC operationtemperature increases.

An additional feature which may be provided to the embodiment of FIGS.1-3 is the addition of a heater 33 to EML PIC 10 as illustrated in FIG.6. The inclusion of heater 33 in close proximity to the PIC activeelements minimizes the temperature excursion of EML PIC 10A. As shown inFIG. 6, EML PIC 10A includes a strip heater 33 along the side of bothintegrated electro-optic elements 12 and 14. Strip heater 33 is athin-film heater which may be a Pt/Ti bilayer, W layer, Pt film, Crfilm, NiCr film, TaN film deposited on the top surface of EML PIC 10Aand can also be any other materials as known in the art for making sucha strip or bulk heater.

DFB laser 12 is designed to be operable over a 40° C. temperature range,such as between, for example, about 30° C. to about 70° C. As previouslyindicated, lasers typically increase in operating wavelength by about0.16 nm/° C. so that their operational wavelength can be changed withina tunable wavelength range of about 4 nm over this temperature range. Inthe application here, heater 33 in FIG. 6 is deployed to heat EML PIC10A, in particular relative to heating laser 12, to its maximumoperating temperature. Then, the ambient temperature is monitored, via amonitoring circuit which includes a thermistor for monitoring thetemperature of laser 12. If the ambient temperature increases above themaximum operating temperature of laser 12, then the monitoring circuitwill decrease a set pre-biased voltage condition of heater 33 whichpermits a return of the laser operating temperature to its maximumoperating temperature or at least within a limited operating temperaturerange. The maximum operating temperature is also within the window ofthe desired operating wavelength for DFB laser 12. As a specificexample, if the operating temperature range of laser 12 is from about40° C. to about 70° C. and the desired wavelength operation of laser 12is approximate to 45° C., then heater 33 would be pre-biased to maintainthe ambient temperature of EML PIC 10A from, for example, from about 40°C. to about 50° C. As a result, the maximum temperature deviation thatwould result would be reduced from about 70° C. to about 20° C. This netchange in operating performance would be small. A 20° C. temperatureexcursion would restrict the DFB laser emission wavelength to about 200GHz which still meets the requirements of CWDM channel spacing whilestill maintaining a long reach optical signal quality.

From the foregoing, it can be seen that the deployment of heater 33, inlieu of a laser bonded TEC, to perform the minimum temperature excursionrelative to the desired application temperature while maintaining anytemperature excursion within the allowed wavelength band for a WDMsignal on the ITU wavelength grid, heater 33 functionally replaces theTEC or other such cooler, which is a comparatively expensive PICcomponent, provides for a larger footprint, and requires a hermeticallysealed package, all which increase the costs of an integrated PIC suchas an optical transmitter, which costs are not generally required in thecase of coolerless EML PIC 10 and 10A of this invention. Thus, heater 33permits the temperature control of laser 12 in a coolerless environmentwithout a substantial need for a hermetically sealed package for EML PIC10A while preserving the required operating laser temperature within thepermissible wavelength band tolerances for channel signals. As notpreviously recognized in the art for a DFB laser or an EML PIC, the useof an integrated heater 33 eliminates the need of a TEC while preservingrequired laser acceptable temperature and wavelength operatingconditions and performance over a high temperature operating range.

In addition to the foregoing temperature tuning, fine tuning with otherapproaches may additionally be included to wavelength tune laser source12, other than or in addition to heating and cooling. For example,employing current tuning via changes to the laser drive current orthrough phase tuning such as in the case where laser source 12 is a DBRlaser and has a phase tuning section.

A further embodiment of a coolerless EML PIC is shown at 10B in FIG. 7which, in addition, comprises a variable gain/loss element 35, whichfunctions as a SOA/VOA, and is integrated in the EML optical path afterEAM 14. In the case of fine tuning the laser wavelength via drivecurrent changes to laser 12, this will also change its output power sothat with such current changes having an accompanying decrease in power,gain/loss element 35 is operated with a positive bias, functioning as aSOA, to increase the power output to a desire maintained power levelthrough via the applied positive bias of element 35. This isparticularly important in an uncooled EML PIC because with increasingtemperature, the power output correspondingly decreases. Also, withincreasing operational temperature of the EML PIC in a coolerlessambient, the current of the laser source may be also decreased tomaintain the operating characteristics. The resulting decrease incurrent is a decrease in output power so that the gain/loss element 35may be operated to increase the signal power to an acceptable level. Bythe same token, if such current changes with an accompanying increase inpower or is operated at a high optimum power to maximize certain laseroperating characteristics, gain/loss element 35 is operated with anegative bias, functioning as a VOA, to decrease the output power of EMLPIC 10B to a desired maintained power level through power absorption viathe applied negative bias of element 35.

Typically, as the temperature of EML PIC 10B, or any previous embodimentfor that matter, increases, the laser-modulator detuning decreases.Although the Q of EAM 14 is improved or maintained constant because ofthis, the power output of EML PIC 10B would decrease or become degraded.The deployment of an integrated power controlling element 35 providesfor design freedom to insure constant output power at elevatedtemperatures while maintaining the Q performance of the EML PIC over thepermitted temperature operating range. In this connection, theembodiments of FIGS. 6 and 7 may be combined to control the operatingcharacters of the laser source 12 and the EAM 14 over a high temperatureoperating range employing both strip heater 33 and element 35. In thisregard, it should be realized that strip heater 33 may be divided intotwo separate parts 33A and 33B, one for laser source 12 and the otherEAM, in order to temperature control these two electro-optic elementsindependently of one another, as will be explained in greater detaillater on in connection with transmitter photonic integrated circuits orTxPICs to which the forgoing principals of coolerless operation may alsobe applied.

The foregoing embodiments have dealt with approaches to operation of acoolerless EML PICs. The principles for coolerless operation are alsoapplicable to an array of coolerless multi-channel PICs comprising amonolithic InP-based chip such as the type illustrated in FIG. 8. FIG. 8discloses a transmitter photonic integrated circuit or TxPIC chip 50which is an In-based chip, the structural details of which are disclosedin incorporated U.S. patent application Ser. Nos. 10/267,330 and10/267,331, supra. In the case here, however, TxPIC chip 50 is operatedin a coolerless mode, which is it is operated without the use of anycooler, such as a TEC. As shown in FIG. 8, coolerless, monolithic PICchip 50 comprises groups of integrated and optically coupled active andpassive components including an integrated array of laser sources 52,such as DFB semiconductor lasers or DBR semiconductor lasers. Each lasersource 52 operates at a different wavelength, λ₁-λ_(N), from oneanother, where the group of wavelengths provides a wavelength gridapproximating a standardized wavelength grid, such as the ITU wavelengthgrid. Such a wavelength grid is illustrated in FIG. 28. As shown in FIG.28, the laser source wavelength grid is provided to have, as best aspossible, a uniform or periodic channel wavelength pitch or an arrayspectral spacing, Δλ, as well as a uniform channel width. However, inone important feature of this invention, to be explained in more detaillater, chip 50 may be permitted to float within a predeterminedtemperature range while the grid or channel spacing remains constant orfixed. In other words, on one hand, the wavelength grid is permitted tochange in wavelength with changes in temperature, which means that theindividual wavelengths of the laser sources are also changing withtemperature but, on the other hand, the array spectral spacing is set toremain the same, such as, for example, in the case of a uniform spacing,set to 25 GHz, 50 GHz, 100 GHz or 200 GHz. The gird spacing can also beprovided with nonuniform spacing which remains fixed.

At the rear extent of laser sources 52, integrated rear photodetectors51 may be provided, which are optional. Photodetectors 51 may be, forexample, PIN photodiodes or avalanche photodiodes (APDs). Laser sources52 may be directly modulated or may be operated CW and are provided withan associated external electro-optic modulator 54 as shown in theconfiguration of FIG. 8. Thus, the CW outputs of laser sources 52 areoptically coupled to respective electro-optic modulators 54 formingchannel modulated sources. Such light intensity modulators 54 may beelectro-absorption modulators (EAMs) or Mach-Zehnder modulators (MZMs)as detailed in patent application Ser. No. 10/267,331, supra, but EAMsare preferred for coolerless operation here in conjunction with DFBlaser sources. Modulators 54 each apply an electrical modulated signalto the CW light received from laser sources 52 producing a plurality ofoptical modulated signals of different wavelengths from the multiplechannels for transmission on an optical link in an optical transport ortransmission network. The modulated outputs from modulators 54 may beoptically coupled to front photodetectors 56. The on-chip deployment ofphotodetectors 56 is optional. Alternatively, photodetectors 56 may alsobe fabricated off-axis of the laser source output by means of an on-chipoptical tap to provide a small portion of the modulated output directedfrom the main optical channel or waveguide path to an offset, integratedphotodetector. Front photodetectors 56 may be PIN photodiodes oravalanche photodiodes (APDs). Photodetectors 51 and 56 may also beemployed together to monitor the output power or operational wavelengthfrom the respective laser sources 52. Alternatively or in addition,photodetectors 56 may also function as variable optical attenuators(VOAs) under negative bias in order to selectively adjust modulatedsource output power to equalize the optical output power across theentire array of laser sources 52 thereby providing on-chip integratedpre-emphasis. Further, alternatively or in addition, photodetectors 56may be employed as on-chip semiconductor optical amplifiers (SOAs) underpositive bias. These devices can, therefore, perform a VOA/SOA functionto provide for power level compensation in the manner discussed inconnection with the PIC embodiment of FIG. 7. Also, as a furtherembodiment, a different frequency tone may be applied to each frontphotodetector 56 to provide for laser source tagging or identificationas described and taught in U.S. patent application Ser. No. 10/267,330,supra.

As indicated above, and as explained in more detail in patentapplication Ser. No. 10/267,331, supra, the modulated optical signaloutputs of modulators 54, via front photodetectors 56, are respectivelycoupled to an on-chip wavelength selective combiner or filter, shownhere as an arrayed waveguide grating or AWG 60 via optical inputwaveguides 58, numbering the number of signal channels of modulatedsources. It is within the scope of this invention to include otherwavelength-selective combiners or decombiners, as the case may be forintended uses, such as, for example, Echelle gratings, cascadedMach-Zehnder interferometers (MZIs), broadband multiplexers of the typeshown, for example, in U.S. Pat. No. 6,580,844 (which is incorporatedherein by its reference), or so-called free-space diffraction gratings(FSDGs). Such wavelength-selective combiners or multiplexers are moreconducive to higher channel signal counts on TxPIC chips 50. However, itis within the scope of this invention to practice the invention inconnection with non-wavelength selective type of optical combiners, suchas power couplers, star couplers, MMI couplers or optical couplers witha multimodal coupled region comprising a plurality of waveguides asdisclosed and taught in U.S. published patent application, PublicationNo. 2003/0012510, which application is incorporated in its entiretyherein by its reference or a multimodal coupled region that is, in part,multiple waveguides and, in part, free-space as disclosed in U.S. Pat.No. 7,745,618, which patent is incorporated herein by its reference.

Each of the modulated sources or, for example, semiconductormodulator/laser (SML) signal channels, or more particularly EML signalchannels, is representative of an optical signal channel on TxPIC chip5, which, for example, may have as many as forty signal channels ormore. In FIG. 8, there is a plurality of N equal 10 channels on TxPICchip 50. There may be less than 10 channels or more than 10 channelsformed on chip 50. In FIG. 8, the output of each signal channel from arespective, integrated EML signal channel is coupled to a respectivewaveguide 58(1) to 58(10) to the zero order Brillouin zone input of AWG60.

It is within the scope of this invention that photodetectors 56 functionas VOAs or SOAs for the purpose of pre-emphasis across the modulatedsource array, which pre-emphasis is different when operating in thehigher temperature range. It is different to the extent that the PICelements may have wider sensitivity (relative to gain or loss) at higheroperating temperatures so that wider dynamic range for setting channelpower may be necessary. Also, as well understood in previouslyincorporated patent applications herein, that photodetectors 51 and 56,laser sources 52 and modulators 54 are electrically isolated from oneanother.

Also, it should be noted that the output capability of each laser source(DFB or DBR) is a sensitive function of the designed laser sourcegrating that sets its emission wavelength from the peak of the activeregion wavelength. Performance gains over temperature can be obtained bydesigning the laser source grating with respect to gain peak such thatalignment between the two improves at higher temperature so that acoolerless TxPIC 50 can be made more of a reality with such a gainadvantage. Particularly, laser sources with relatively larger detunedwavelengths can take good effect of this advantage although there canbe, in some instances, a rise in the laser source threshold current.Also, the high temperature operation of the TxPIC laser sources does notaffect their single mode properties as seen from their side modesuppression ration (SMSR) even at about 70° C., being close to 40 dB.The total tuning rate of DFB lasers as on-chip laser sources 52 is about0.16 nm/° C. or −20 GHz/° C. On the other hand, the tuning rate of anAWG as an on-chip combiner is about −16.25 GHz/° C. If the DFB laserbias remains unchanged, then it's natural tuning rate is lower than 20GHz/° C. and will be closer to the AWG tuning rate. In any case, thedifferential tuning between a DFB laser at constant power and an AWG isabout 3.75 GHz/° C., which is fairly small so that the co-thermaltracking and control can be easily realized, which is a basic objectiveof this invention, to maintain a substantial grid alignment between thelaser source wavelength grid with the optical combiner passband over theentire high temperature operating range. Also, the insertion loss of anAWG over the higher temperature operating range is fairly constant.However, changes in local operating temperatures of the laser sourceswith the combiner may have a detrimental effect on associated modulatorsin signal channels between these two on-chip elements (between a lasersource and AWG or combiner). As the ambient temperature of the TxPICincreases, the wavelength detuning of a laser source relative to PL peakof the active region wavelength is reduced so that the bandedge of theactive region red shifts faster than the laser source operatingwavelength. This leads to an increase in the on-state loss of a channelEAM, or its overall insertion loss, as the absorption edge moves closerto the laser source signal or operating wavelength. Such insertionlosses are preferably not tolerated because the optical signal reach inan optical fiber of the TxPIC transmitter is substantially affected.Thus, it becomes necessary in such a changing temperature environment toadjust the effective chirp parameter of the EAM so that the modulator isheated to move the EAM absorption edge further away from the lasersource operating wavelength. In this case, it should be noted that theheater for the laser and the modulators need to be separate heaters.Also, in order to minimize the complexity of optimizing the performanceof the EAMs under changing temperature conditions, it is preferred thatthe modulator swing voltage is kept constant and the modulator biasvoltage be varied with changes in temperature of the modulator.

As already indicated earlier, each signal channel is typically assigneda minimum channel spacing or bandwidth to avoid crosstalk with otheroptical channels. For example, 50 GHz, 100 GHz or 200 GHz are commonchannel spacings between signal channels. The physical channel spacingor center-to-center spacing 68 of the signal channels may be 100 μm, 200μm, or 250 μm to minimize electrical or thermal cross-talk at higherdata rates, for example, of 10 Gbit per second or greater, andfacilitate routing of interconnections between bondpads of multiple PICoptical components or elements formed on the chip. Although not shownfor the sake of simplicity, bonding pads may be provided in the interiorof PIC chip 50 to accommodate wire bonding to particular on-chipelectro-optic components in addition to chip-edge bonding pad groups 55.

Referring again to combiner 60 comprising an AWG, the respectivemodulated outputs from electro-optic modulators 52 are coupled intooptical waveguides 58(1) to 58(10) to the input of AWG 60 as shown inFIG. 8. AWG 60 comprises an input free space region 59 coupled to aplurality of diffraction grating waveguides or arms 61 which are coupledto an output free space region 62. The multiplexed optical signal outputfrom AWG 60 is provided to a plurality of output waveguides 63 whichcomprise output verniers along the zero order Brillouin zone at outputface 62A of free space region 62. Output waveguides 63 extend to outputfacet 69 of TxPIC chip 60 where a selected vernier output 63 may beoptically coupled to an output fiber (not shown). The deployment ofmultiple vernier outputs 63 provides a means by which the best oroptimum output from AWG 60 can be selected having the best match of thewavelength grid passband of AWG 60 within a range of different operatinghigh temperatures with the established wavelength grid of the lasersources. Seven vernier outputs 63 are shown in FIG. 8. It should berealized that any number of such vernier outputs may be utilized. Also,the number of such vernier outputs may be an odd or even number.

In operation, AWG 60 receives N optical channel signals, λ₁-λ_(N), fromcoupled input waveguides 58 which propagate through input free spaceregion 69 where the wavelengths are distributed into the diffractiongrating arms or waveguides 61. The diffraction grating waveguides 61 areplurality of grating arms of different lengths, by ΔL, from adjacentwaveguides, so that a predetermined phase difference is established inwaveguides 61 according to the wavelengths λ₁-λ_(N). Due to thepredetermined phase difference among the wavelengths in grating arms 61,the focusing position of each of the signals in grating arms 61 inoutput free space region 62 are substantially the same so that therespective signal wavelengths, λ₁-λ_(N), are focused predominately atthe center portion or the zero order Brillouin zone of output face 62A.Verniers 63 receive various passband representations of the multiplexedsignal output from AWG 60. Higher order Brillouin zones along outputface 62A receive repeated passband representations of the multiplexedsignal output but at lower intensities. The focus of the grating armoutputs to the zero order Brillouin zone may not be uniform along face62A comprising this zero order due to inaccuracies inherent infabrication techniques employed in the manufacture of chip 50. However,with multiple output verniers, an output vernier can be selected havingthe best or optimum combined signal output in terms of power andstrength.

PIC chip 50 with its integrated array of N modulated sources can beoperated coolerless as taught in earlier embodiments with regard to EMLPICS. The active region of chip 50 may comprise AQ to provide for awider, substantially stable temperature window across the modulatedsource array comprising signal channels 1 to 10 as illustrated in theFIG. 4 for a single EML PIC. With the deployment of an AQ active region,there is little change in output power of the laser source array acrossthe chip over a wide temperature range of operation, such as from about40° C. to about 70° C. with proper detuning. Also, in combination withthe utilization of an AQ active region across the chip, laser source 12is positively detuned, i.e., the grating pitch of the feedback gratingof the respective DFB lasers 52 are chosen such that the laser operateson the longer wavelength side of the gain peak. This detuning providesfor laser performance to be substantially uniform over a wider widetemperature range, in particular, the laser gain is maintained oractually increases some with increasing operating or ambient temperatureas previously discussed. Laser sources 52 may be fabricated to operateat a respective positive detuned wavelength, for example, in the rangeof about 25 nm to about 40 nm from the gain peak. The laser detunedtransmission wavelength is close to the absorption edge of the modulatorAQ active waveguide core insuring optimal wavelength compatibilitybetween laser sources 52 and corresponding EAMs 54 without significantlydegrading the performance of the laser sources due to the appliedpositive detuning.

In addition, chip 50 may include strip heaters 53A formed adjacent to orin close proximity to each laser source 52 as shown in FIG. 8. Theseheater are employed to fine tune the operating wavelengths and,therefore, the result wavelength spacing, between adjacent laser sourcesin the array. Strip heaters 53B may also be employed adjacent to or inproximity to EAMs 54. In the case of MZMs on chip 50, a heater 53B wouldbe placed along a portion of each arm of the Mach-Zehnderinterferometer. Also, as shown in FIG. 29, the EAM structure 15, whichmay be part of a modulated source in each of the signal channels in FIG.8, includes on top a dielectric layer 35, which may be, for example,Si₃N₄, over which is longitudinally formed, along the modulator length,a heater strip 37. Heater 37 may be comprised of a Pt/Ti bilayer, Wlayer, Pt film, Cr film, NiCr film, TaN film deposited on the topsurface of dielectric 35. Having the heater 37 on top of each EAM 15 or54 is the most efficient for transfer of heat because the heater is onlyapproximately 1 μm away from the active region 26 of the modulator. Thisis more efficient than the heater positions suggested, for example, inU.S. Pat. No. 6,665,105, which are not as effective and are difficult tomanufacture. Heaters 37 or 53B are employed to optimize the operation ofmodulators 15 and 54 due to wavelength changes made to the wavelengthgrid of the laser array. Operating characteristics of EAMs can becomeoffset from an optimized condition due to thermal changes, andtherefore, wavelength, of its corresponding laser source 12 or 52. Thus,the temperature of the modulators should be monitored since the EAMbandgap offset from the detuned emission wavelength of the laser sourcescan change with operating temperature changes to the laser sources. Themonitored temperature is used to vary the modulator DC bias voltage withtemperature. Also, the DC bias voltage of the modulators will have to beadjusted relative to changes in modulator absorption due to temperaturechanges to achieve optimum modulator performance. Thus, heaters 53B maybe utilized to independently adjust the EAMs to optimize thereabsorption and bandedge as well as modulator chirp or adjustment oftheir absorption with applied bias to optimize their extinction ratio(ER). Further, a heater 60A may be provided for combiner 60 as indicatedin FIG. 8. Since the tuning or slew rate between DFB lasers sources 52and combiner 60 are approximate, the wavelength grid can be concurrentlytuned to maintain a approximate grid relationship with one other. Thisheater 60A may, for example, be a serpentine metal strip formed over thecombiner area of the chip. The heater serpentine strip may be comprisedof a Pt/Ti bilayer, W layer, Pt film, Cr film, NiCr film, TaN filmdeposited on the top surface of a dielectric, such as Si₃N₄, formed overthe area of combiner 60. Also, it is with the scope of this invention touse a heater across the grating arms 61 of AWG multiplexer 60 to controlthe center wavelength of AWG 60 to maintain a substantial grid match towavelength grid of laser sources 52. An example of the foregoing heatedgrating arms is seen in U.S. Pat. No. 5,617,234, which patent isincorporated herein by its reference.

It is also within the scope of the embodiment of FIG. 8 to include powerchanging elements (PCEs) in each channel between EAM 54 and front PD 56.Such a PCE may be a SOA, VOA or a combination SOA/VOA. The PCE may alsohave an accompanying heater to move the gain peak or to move the gainpeak in tandem with laser source gain peak.

As indicated previously, the heaters 37 and 53B are employed in thecontrol of the modulated sources can be accomplished without therequirement of a TEC or Peltier cooler when changes in temperature aremade by heaters 53A and 60A to maintain channel spacing of signalchannels required for coolerless operation of TxPIC chip 50, therebyeliminating the expensive cooler component. The use of these heaters 50and 60A to stabilize the operating temperature of the laser sources 52within an acceptable temperature range, rather than using a TEC, reducespackaging costs significantly as well providing for a smaller footprintand reducing if not eliminating the requirement for a hermeticallysealed chip package, resulting in a coolerless TxPIC transmission chip.

Reference is now made to FIG. 9 which shows the typical layout for anRxPIC 40. It should be noted that RxPIC chip 40 is just one embodimentof many that may be employed in a digital optical transmission network.See, for example, the different embodiments illustrated in U.S. patentapplication Ser. No. 10/267,304, which is incorporated herein by itsreference. A particular example is the provision of an integratedoptical amplifier (OA) 42 on RxPIC chip 40, such as a semiconductoroptical amplifier (SOA) or a gain-clamped semiconductor opticalamplifier (GC-SOA). RxPIC 40 is an InP-based semiconductor chip that hasan input at 41 to receive a multiplex optical signal from an opticallycoupled fiber link. Optical amplifier 42 may be integrated in thecircuit to boost the gain of the multiplexed signal prior todemultiplexing. Such amplification can alternatively be done off-chipwith an optical fiber amplifier positioned before the input of the WDMsignal into on-chip waveguide 41. The multiplexed signal is received inchip waveguide 43 and provided as an input to decombiner 44 which may,for example, be an AWG. The multiplexed signal is provided to input slabor free space region 46A of AWG 44. AWG 44 comprises input slab 46A, anarray of grating arms 44A of different lengths and an output slab 46B asknown in the art. Output slab 46B has a plurality of outputs in thefirst order Brillouin zone, one for each demultiplexed channelwavelength signal, which are respectively provided to PIN photodiodes49(1) . . . 49(12). Again, although there are twelve channels shown herefor chip 40, there may be as many as 40 or more such channel signaloutputs from AWG 44 with corresponding photodetectors 49. A higherBrillouin order output channel at 47A may also be provided on RxPIC chip40 to provide a channel light output to PIN photodiode 48 in order tomonitor the wavelength, power of the signals or provide for FECdecoding. Also, to be noted relative to the present invention,decombiner 44 also has a local heater 45, which is similar to heater 60Ain FIG. 8. This heater 45 may, for example, be a serpentine metal stripformed over the decombiner area of the chip. The heater serpentine stripmay be comprised of a Pt/Ti bilayer, W layer, Pt film, Cr film, NiCrfilm, TaN film deposited on the top surface of a dielectric, such asSi₃N₄, formed over the area of decombiner 44.

Reference is now made to FIG. 10 illustrating an embodiment for carryingout this invention employing the foregoing coolerless TxPIC and RxPICchips 40 and 50 in a floating grid, optical transmission WDM networkoperating under the conditions that the comb of operating wavelengthgrid of the modulated sources “floats”, meaning that the operatingmodulated source wavelengths are permitted to drift with variations inambient temperature within a given temperature range or the modulatedsources can be heated to a maximum temperature within a high temperatureoperating range for the PIC but the given or predetermined wavelength orarray spectral spacing between adjacent modulated sources in the lasersource array is maintained at a fixed value, i.e., the comb ofwavelengths of an TxPIC laser array are locked to a fixed frequencyspacing where the array spectral spacing between any two laser sourcesin the TxPIC may be uniform (all the same bandwidth) or nonuniform(different bandwidths including one or more different from all theothers in the grid or monotonically increasing or decreasing inbandwidth between adjacent laser sources across the grid). The floatingwavelength grid, made up of N signal channels, can change in wavelengthup or down within a given wavelength bandwidth according to apredetermined operational high temperature range as the ambienttemperature of the PIC changes but the laser source array wavelengthspectral spacing between the signal channels remains fixed. The TxPICchip is allowed to operate in a higher temperature environment, such as,for example, between room temperature and 70° C. or more, such aspossibly as high as 85° C.

This floating wavelength grid approach is contra to SONET/SDH standardswhere the signal channels are maintained along a standardized ITUwavelength grid. However, certain advantages are achieved through thedeployment of this floating grid approach. First and foremost, theadaptation of multiple signal channels on a single PIC chip lends itselfto better temperature control of active and passive components orelements on the chip rather than attempts at stabilizing the chipambient via a TEC cooler. Second, a temperature stabilization system fora PIC chip is extensive including a costly chip cooler and it would beless expensive and easier to operate a PIC chip in a high temperatureenvironment including high temperature hazardous environments formilitary deployment. Third, the TxPIC package does not generally need tobe a hermetically sealed package.

The floating grid optical transmission network shown in FIG. 10comprises TxPIC chip 100 optically linked in an optical point-to-pointtransmission system via optical link 119 to RxPIC chip 120. TxPIC 100comprises a plurality of integrated components in plural paths of Nsignal channels, identified by λ₁ to λ_(N), to AWG multiplexer 110 whereeach such path includes a laser source 102, shown here as a DFB laser,an electro-optical modulator 106 and a SOA or VOA 108 coupled to a firstorder input of AWG 110. Each laser source 102 is operated cw at a peakwavelength different from other sources. The output of each laser source102 is modulated with an information signal at its respective modulator106(1) . . . (N). Modulators 106 may be, for example, a semiconductorelectroabsorption (EA) modulator or a Mach-Zehnder (MZ) modulator aspreviously explained. The modulated signal may then be provided withadditional gain or attenuation via SOA or VOA shown as power changingelement (PCE) 108. SOAs and/or VOAs 108 are optional. Alternatively,PCEs 108 may be forward photodetectors (FPDs) for monitoring powerand/or wavelength as well as operating as a PCE, such as a VOA. Theoutputs from elements 108 are provided as inputs to AWG multiplexer 110.The combined WDM output of AWG multiplexer 110 is optically coupledoff-chip to optical link 119.

In order to operate TxPIC chip 100 in a coolerless mode, each DFB source102 is provided with a corresponding integrated heater 102A and eachmodulator 106 is optionally provided with a corresponding heater 106A.Also, AWG 110 is optionally provided with a heater 110A. The DFB heaters102A are for fine tuning of the laser wavelength to maintain properwavelength grid channels spacing relative to adjacent signal channels.The modulator heaters 106A are to maintain the absorptioncharacteristics of the modulators with optimum extinction ratio andbias, as the operating characteristics of the laser sources 102, detunedfrom the gain peak of the active region wavelength, will also beaffected with changes in temperature, which also affects the performanceof the modulators. The third heater 110A for AWG 110 maintains thealignment of the AWG wavelength comb or grid with the wavelength comb orgrid of laser sources 102. The heater 110A may be a serpentine stripheater over the AWG 110 and separated therefrom by a dielectric layer.It is also within the scope of this invention for heater 110A to beformed adjacent to AWG 110.

A small sample of the multiplexed channel signal output from AWG 110 isprovided through an optical tap at the multiplexed signal output fromAWG 110 to photodiode (PD) 112 which provides a photocurrent input toprogrammable logic controller (PLC) 116. PLC 116 discriminates among thedifferent channel signals, λ₁ . . . λ_(N), to determine if the operatingwavelengths of DFB sources are at their desired emission wavelengths forproper frequency or spectral spacing as detuned from the peak activeregion wavelength. This discrimination process can be carried out byemploying dithering signals on the modulated channel signals for eachmodulated source on TxPIC 100, providing each such signal with anidentification tag. As a result, each of the channel signals can beseparated and analyzed as to its wavelength to determine if it isoperating at a desired, fixed channel spectral spacing, as seen in FIG.28, relative to adjacent signal channels as well as optionally operatingsufficiently close to its desired peak channel wavelength within achannel bandwidth. If the channel spacing of any particular laser source102 is off from a desired and fixed channel spacing relative to anadjacent signal channel, its operating wavelength can be changed to thedesired grid wavelength spacing by a signal provided from PLC 116 toheater control circuits (HCCs) 120A and 120B which provides atemperature control signal to a corresponding laser source heater 104Afor fine tuning, such as a few nanometers or tenths of nanometers, byincreasing or decreasing the operating temperature of its correspondinglaser source 102 by an amount necessary to increase or decrease itsoperating wavelength bandwidth to be substantially at the desiredchannel spectral spacing. Although the current of laser sources 102(1) .. . 102(N) may be adjusted for power control, this not as desirablebecause of the accompanying wavelength tuning that occurs. The preferredapproach is to bias laser sources 102 at the highest possible currentlevel, within the limits of reliability and desired operatingwavelength, and employ an on-chip VOA, or a front photodetectorfunctioning as a VOA or other PCE at 108, to compensate for power lossas a result of misalignment between the laser source wavelength comb orgrid with the combiner wavelength grid or passband or due to powerdegradation of laser sources 102 with temperature or due to aging. Insuch a preferred approach, the above mentioned fine tuning of lasersources 102 via their heaters 102A is a valuable asset in TxPIC highperformance operation. In this manner, heaters 102A can also be deployedto tune the operating wavelength of laser sources 102(1) . . . 102(N) tokeep the laser source array operating as a floating channel grid withfixed channel spacing. Thus, heaters 102A provide an ability to performfine wavelength tuning over a given temperature range. The same is truerelative to heaters 53A and 53B in the embodiment of FIG. 8.

It should be noted that the tuning rate via heaters 53A and 102A isfairly constant, about 1 GHz/mW between about 20° C. and about 70° C.and possibly as high as 85° C. This wavelength tuning is also linearwith respect to heater power dissipation. In order to lock the laserwavelength grid to a desired floating grid spectral spacing, theemission wavelength of the respective laser sources must be known withinthis given temperature range. As a specific example, if the tuning ratesof DFB lasers in an array are about −20 GHz/° C., then for a TxPIC witha channel spectral spacing of 200 GHz translates into a temperaturerange of about 10° C. With the knowledge of the tuning rate of the lasersources, a coarse tuning of the laser sources can be achieved fromcontrolling the temperature of the TxPIC chip 100 employing a thermistor(not shown) on the carrier for the TxPIC chip as described in thepreviously mentioned and incorporated patent application Ser. No.10/267,330, supra. Using a lookup table in controller (PLC) 116, forexample, the temperature of the laser sources can be inferred from thechanging resistance value of the thermistor.

As previously indicated, photocurrent from RPDs 101 may be independentlyemployed to measure the laser source output power. A small part of theTxPIC output from TxPIC 100 is tapped of and provided to Fabry-Perotwavelength locker (FPWL) operating with an etalon to provide both anindication of the average output power of TxPIC 100 as well as anindication of the average power and wavelength of the individual signalchannels employing a different low frequency tone on each of the channelsignals in the manner as explained and set forth in the previouslymentioned and incorporated patent application Ser. No. 10/267,330,supra. Controller 116 then provides the following feedback correctionsignals to adjust the following parameters: (1) Adjust laser source biascurrent to the highest reliable output power level for the lasersources. This will also change the operating wavelength of therespective laser sources. (2) Adjust the laser heater current for finetuning, i.e., to adjust for laser source wavelength drift over time andfor wavelength changes with changes in the laser source bias current andtemperature. (3) Adjust FPD (VOA) 108 bias level for TxPIC pre-emphasis,i.e., output power flattening across the N signal channels across TxPICchip 100.

In another embodiment of this invention, the use of a thermistor can bereplaced by the employment of the integrated rear photodetectors (RPDs)101(1) . . . 101(N) and the integrated front photodetectors (FPDs) atpositions indicated at (108(1) . . . 108(N). The ratio of the FPD 108 toRPD 101 is a good indicator of TxPIC temperature. FPDs (VOA) 108 whichhave modulators 106 between it and laser source 102, the photocurrentfrom the FPDs 108, compared with that from RPDs 101, is affected more byTxPIC temperature. FIG. 30 is a graphic, semi-log illustration of theratio of FPD 108 to RPD 101 for the average ratio between channels 1 and10 on an N=10 TxPIC 100. The ratio of the photocurrents is approximatelyexponential with temperature for all channels. In this connection,reference is made to graphic, semi-log linear illustration of FIG. 31which shows the relationship between laser source emission wavelengthfor a 10 channel TxPIC 100 over a wide temperature range with respect tothe ratio of photocurrents from their respective FPDs 108 and RPDs 101.Except at the limits of temperature range control, the emissionwavelengths of the laser sources are substantially linear with theFPD/RPD photocurrent ratio. This information can then, inter alia, beemployed for fine tuning of laser sources 102. One of the advantages ofusing the FPD/RPD photocurrent ratios for controlling laser sourcewavelength is that these devices have a large dynamic range for purposeof implementing the control. The sensitivity can be further improved bymaking the integrated, fabricated lengths of FPDs 108 different from theintegrated, fabricated lengths of RPDs 101.

Optionally, the temperature of AWG 110 at TxPIC 100 may be monitoredwith a thermistor 113 which provides PLC 116 with current information ofthe AWG ambient temperature via input 115. PLC 116 can then provide acontrol signal to heater control circuit (HCC) 118 to provide atemperature control signal to heater 110A to increase or decrease theambient temperature of AWG 110. In this manner, the wavelength passbandgrid of AWG 110 may be shifted and adjusted to optimize the wavelengthgrid or passband of AWG 110 with the floating wavelength comb of N lasersources 102.

Also, the input side of AWG 110 includes a port 117 relative to a higherorder Brillouin zone of the input side of AWG 110 for the purpose ofreceiving a service signal, λ_(s), from RxPIC 120 via optical link 119,which is explained in further detail below. This service signal isdemultiplexed by AWG 110 and provided on port 117 as an output signaland thence converted to the electrical domain by integrated, on-chip PD114. The electrical signal from PD 114 is taken off-chip and provided asan input 119 to PLC 116.

At RxPIC chip 120, AWG demultiplexer 123 includes higher order Brillouinzone outputs 125A and 125B to receive respective channel signals, suchas, for example, λ₁ and λ₂ or any other such signal pairs, in order todetermine the position of the floating wavelength grid or comb receivedfrom TxPIC 100 via link 119 within a determined range of wavelengthsdetermined by a temperature range over which the wavelength grid ispermitted to float. Also, using these two channel signals as awavelength grid sample, a determination can be made as to whether theAWG wavelength is shifted and, if so, by how much. Photodetectors 125Aand 125B provide an electrical response to optical signals on outputlines 126A and 126B to programmable logic controller (PLC) 127. ThesePDs 125A and 125B are sensitive to the peak optical responses of thetotal grid output and can be deployed in the electrical domain todetermine the spectral location of the floating wavelength grid in orderto lock onto the grid and then demultiplex the lock grid of channelsignals and convert them into electrical signals via the integratedphotodetectors 126(1) . . . 126(N) on chip 120. Also, if the deltashift, δ, of the signal grid is detected as either a red shift or a blueshift, a delta shift value can be provided back TxPIC 110 via a servicechannel, λ_(OSC), for purposes of aligning subsequent transmittedchannel signal grids from the transmitter chip 110 more in thermalalignment with immediately received channel signal grids at the receiverchip 120. Receiver PLC 127 can first make adjustment to the receiver AWGwavelength grid, via heater control circuit (HCC) 130 via line 132 toAWG heater 123A, to either increase or decrease the ambient operatingtemperature of AWG 30 and to shift its wavelength grid either to thelonger or shorter center wavelength to match the floating grid ofincoming channel signals based on the determined delta shift, δ, of theWDM signal floating grid. If this grid adjustment is not sufficient,then data relating to channel signals floating gird may be forwarded asa service channel signal, λ_(s), for thermal adjustment at thetransmitter end of the floating wavelength grid. In these circumstances,PLC 127 can forward such grid correction data as a service channelsignal, λ_(s), via an electrical correction data signal on output line128 to service signal channel modulator 129, which may be comprised ofan on-chip, combination integrated laser source and an electro-opticmodulator, to provide this signal through AWG 123 and counterpropagation via optical link 119 service channel to TxPIC chip 100 atthe transmitter end of the network. This service channel signal, λ_(s),is then demultiplexed via AWG 110 and provided on higher Brillouin orderoutput 117 to PD 114. The electrically converted service signal data isdeciphered by PLC 116 which makes a correction to the thermal ambient ofa laser sources 102 via HCC 120A along with correction to the thermalambient of AWG 110 via HCC 118, if necessary. As will be seen below, thethermal ambient of laser sources 102 and AWG 110 are maintained to besubstantially the same since the rate of change in thermal properties,and as a result a change in wavelength grid match-up, approximate oneanother. This process may optionally also involve changing of thecurrent level of sources 102 as well as the bias level and extensionratio of the corresponding modulators 106A by changing the bias levelchanges from PLC 116 as well as changing their operational temperaturevia heaters 106A. Also, front photodetectors (FPDs) at 108(1) . . .108(N) may be operated as a power changing element to provide on-chippre-emphasis due to changes in operating current levels of laser sources102 in adjust of the floating wavelength comb or grid formed by theselaser sources.

FIG. 11 illustrates a further embodiment for carrying out this inventioncomprising floating grid network 140. Network 140 includes on thetransmitter end at least one TxPIC 142 and at least one RxPIC 144 whichare coupled to optical transmission link 146. TxPIC 142 includes aplurality of N signal channels of laser sources LD(1) . . . LD(N),modulators M(1) . . . M(N) and photodetectors PD(1) . . . PD(N).Alternatively, as discussed in the embodiment of FIG. 10, thephotodetectors may alternatively be power changing elements (PCEs). Thesignal outputs of N channels are coupled as inputs to combiner 148,which is shown here as a wavelength selective combiner, which combinerprovides a combined WDM signal onto link 146. As in the embodiment ofFIG. 10, TxPIC 142 includes a feedback at tap 149 comprising a portionof the WDM output signal which is provided to programmable logiccontroller (PLC) 150. At PLC 150, the optical feedback signal from tap149 is converted into electrical signals used in wavelengthidentification in a manner known to those skilled in the art. PLC 150also has electrical signal outputs to heaters 141 for N laser sources,electrical signal outputs to heaters 143 for N modulators and anelectrical signal output to heater 145 for combiner 148.

On the receiver end, a WDM signal is received from link 146 by RxPIC 144via an optical amplifier 155 at the PIC input. RxPIC 144 comprisesdecombiner 147 m shown here as a wavelength selective decombiner, whichhas H optical signal outputs, one each to a respective on-chipphotodetector (PD) 151 for OE conversion of the demultiplexed signals.The converted signals are amplified at transimpedanceamplifier/automatic gain control circuits 152 and, thereafter, clock anddata recovery is performed at CDR circuit 154 as known in the art. Also,all of the N signals from circuits 152 are summed at summer circuit 156and the summed value is provided to receiver programmable logiccontroller 158. As can be seen in FIG. 11, an output of PLC 158 isprovided to control the temperature of heater 153 of decombiner 153.

As shown at the top of FIG. 11, TxPIC 142 provides a floating wavelengthgrid of combined signals 157 having a grid center wavelength, forexample, at a given temperature, T₁. TxPIC 142 is not provided with anycooling mechanisms but rather is temperature controlled through theapplication of heater control signals to on-chip heaters 141, 143 and145. In this connection, the temperature control may extend into a hightemperature range, such as between around room temperature to around 70°C. or more and the operation of TxPIC can be set at a maximum operatingtemperature within this high temperature range. In any case, whetherTxPIC 142 is operated without any applied maintenance temperature orwith an applied maintenance temperature, any wavelength shift of channelwavelengths due to changes in the ambient temperature, preferably withina given temperature range, which shift is indicated by arrows 157A atopFIG. 11, is permitted but the spectral spacing between adjacent signalchannels is maintained at a fixed value as previously explained.However, as a result of temperature floating of the laser sourcewavelength grid, the floating wavelength grid may be received at RxPIC144 with a different grid center wavelength that has wavelength-shiftedbecause of a different ambient temperature, T₂. Through communicationbetween PLC 150 and 158, as indicated by the dashed line 160, PLC 158can shift the wavelength filtering comb of decombiner 147 to recognize(detect) the comb of shifted channel wavelengths having a centerwavelength at temperature, T₂, and lock onto the detected wavelengthgrid.

In general, the method of operation in the embodiment of FIG. 12 entailsthe summing of all the decombined signal values at 156 from RxPIC 144which is received by PLC 158 and used as a means of reference todetermine the position, temperature-wise, of the floating wavelengthgrid within a predetermined bandwidth. For this purpose, the summedvalue may be employed by controller 158 at the receiver side whichincludes a lookup table to determine the value of thermal incrementrequired for the receiver decombiner heater 153 to achieve a locked oncondition of the incoming floating wavelength grid by means ofcontrolling the temperature of a demultiplexer heater 153 via PLC 158.In other words, controller 158 tunes the receiver decombiner filter gridto match the floating grid so that an intelligent decombining ordemultiplexing of the N channel signals in the received WDM signal canbe realized.

FIG. 12 illustrates another embodiment for carrying out this inventionrelative to a floating wavelength grid of a WDM signal received on thereceiver side at RxPIC 162. TxPIC 142 on the transmitter side is thesame as TxPIC 142 in FIG. 11. However, on the receiver side, RxPIC 162is different in the manner in which the floating wavelength grid isdetected at the receiver. RxPIC 162 includes at least one broadbandtunable grid filter 164 which may be, for example, an arrayed gratingarm comb filter or other wavelength selective filter. Filter 164 isdeployed to lock onto the WDM signal comb 157 received over link 146 bymeans of employing filter heater 165. Thus, filter 164 may be any gridfilter that is capable of being adjusted in some manner to adjust thewavelength comb of the filter 164 to detect and be centered onto thesignal comb 157.

After filter 164 is able to lock onto wavelength comb 157, the WDMsignal is then decombined at wavelength selective decombiner 166. Thedecombined outputs are then provided to photodetectors 168 for OEconversion. The incoming signal comb 157 also includes, such as an OSCsignal or data in the signal header, a reference signal, λ_(R), thatprovides an indication to PLC 169 of a reference key to the expectedcenter temperature, T₁, of signal comb 157 as well as the fixedwavelength comb spectral spacing between channel signals. Alternatively,this signal can be a starting center wavelength signal, λ_(C),indicative of the center wavelength of signal comb 157 relative totemperature, T₁, at TxPIC 142. Based upon one of these informationsignals, PLC 169 can shift the filter comb of filter 164 via heater 165and thereafter lock onto the discovered grid which may have a new combcenter wavelength, for example, at temperature, T₂.

The method of operation of the FIG. 12 embodiment entails, first,communication of the comb spectral spacing of the signal channels fromTxPIC 142 and a wavelength reference signal, λ_(R), or a startup centerwavelength reference signal, λ_(C), of the grid. Second, thedetermination of the grid position within a known signal bandwidth atRxPIC 162 based upon the signal, λ_(R) or λ_(C), about the transmitterwavelength comb and locking onto the detected grid or comb of thereceived WDM signal. Third, adjust of the wavelength grid position ofthe receiver channel signal decombiner 166 based upon the discoveredgrid accomplished at filter 164. Fourth, decombine or demultiplex thechannel wavelengths via one or more combiners 166 for conversion intoelectrical domain signals. Fifth, communicate back to TxPIC 142 that thedetermined grid position has been achieved and, if desired, a referencesignal, λ_(R), indicative of the instantaneous locked-on position of thefloating wavelength grid at the optical receiver which can be sent tothe optical transmitter indicating that a lock on the transmittedfloating wavelength grid has been achieved. Rather than sending awavelength reference signal, λ_(R), in an optical service channel (OSC),the transmitted signal frames may designate a reserved byte or otherbyte in the frame overhead to contain information relating to thereference or control signal information.

In another approach relative to the embodiment of FIG. 12 is that, atstartup of the communication exchange between TxPIC 142 and RxPIC 162 ina corresponding transmitter and receiver, a frequency key, λ_(K), forthe floating wavelength grid is transmitted from the optical transmitterto the optical receiver so that the receiver PIC can track the grid byknowledge of the key. As one example, the key on the transmitter sidemay be a set value based from a lookup table at the transmittercontroller correlated to the instantaneous center wavelength of thetransmitter floating wavelength grid. Since at startup, there may sometransients, in any case, the tracking by a frequency key can becommenced until the optical receiver can lock onto a wavelength grid ofan incoming test or correlation signal which is indicative that thetemperature tracking between the floating grid at the transmitter andthe floating grid identified at the receiver are basically matched. Atthis point, a handshake can be established by the receiver by sending anacknowledgement to the optical transmitter that a lock-on state has beenachieved so that client channel signals can now be transmitted. Bytuning the wavelength grid of the optical receiver decombiner throughshifting of its wavelength grid according to the frequency key, the gridcan be continuously changed to permit proper demultiplexing ordecombining of the combined channel signals receiver from the opticaltransmitter. Then, tracking can be continued on a continuous basis, suchas on-the-fly, between the transmitter and the receiver as it would beexpected that the movement with time of the floating grid withtemperature would move in a slower and more gradual manner renderingsuch tracking easier. In one example, the tracking at the receiver couldbe accomplished by an BER feedback system in communication with thetransmitter.

As indicated previously and presupposed in FIGS. 11, 12 and 26, thefrequency or channel spectral spacing between adjacent laser sources ina wavelength grid form on a TxPIC may be uniform or periodic, i.e.,substantially identical across the laser array, or array spectralspacing between adjacent laser sources in a wavelength grid form on aTxPIC may be nonuniform aperiodic, i.e. change (increase or decrease) inspectral width monotonically across the array, or some adjacent lasersources in the array may be one spectral width while others in the arraybe different spectral width. In either case of a periodic or aperiodicgrid, only two detected channel wavelengths form together a key tolocate the grid within a given signal wavelength band dependent upon theallowed temperature swing of the coolerless TxPIC. Once the twowavelength keys are simultaneously discovered, the grid can be lockedsince the other grid wavelengths will be automatically discoveredbecause their fixed relationship with the two frequency keys. Thedemultiplexer is tuned or can be tuned to demultiplex theauto-discovered grid. In another embodiment, it can be seen that oneapproach to achieve this autodiscovery is to start the receiverdemultiplexer at the permitted low end of the plausible TxPICtemperature range and then incrementally heat the demultiplexer underthe control of the optical receiver controller or PLC until there is asimultaneous match to the two frequency keys. Such a function is mostuseful during startup to initially match the demultiplexer grid to theincoming transmitter WDM channel signal grid. The temperature changesaffecting the channel wavelength at the TxPIC at this point should be ata much smaller granularity level. This keying function approach may bethe most viable approach since concurrent detection of two spatialchannel wavelengths or two reference wavelengths within the channelsignal grid will then lock to the entire channel grid since the channelfrequency or spectral spacing is fixed.

In another embodiment related to keying, just explained in the previousembodiment employing frequency keying, tone keying can be deployedinstead of such frequency keying. Low frequency tones, such as modulatedsignals in the tens of KHz range, can be employed as signal channelidentification tags on signal channels. These low frequency signalchannel tags do not interfere with their high modulated frequency signalin the Gigabit range because they are so far afield in frequency domainas to be transparent to one another. Such tone signals can besuperimposed on channels, for example, either optionally at therespective laser sources on the TxPIC or at the front photodetectors orat the SOAs or VOAs of each channel, as the case may be. Examples ofthese kinds of tone channel identification tags are disclosed in theincorporated U.S. patent application Ser. No. 10/267,330. In thisembodiment, the transmitter also sends these tones to the receiver as akey for purposes of detection of the channel grid to be transmitted tothe receiver. The tone key can be sent in the signal frame overhead atstartup or as a OSC signal. The advantage of deploying such a lowfrequency tone key is that the receiver can easily identify the toneseven if they are as much as 40 dB down, meaning that the transmitterfloating channel grid has moved quite a bit. A circuit can be deployedat the receiver to discriminate among the different tones using aFabry-Perot discriminator technique, as known in the art, and based uponthe detected tones, move the filter spectrum of the broadband tunablegrid filter to detect the grid and the grid bandwidth based upon thetone key. Such filter spectrum shifting can be accomplished bythermal-optic effect, electro-optic effect, or refractive index changeeffect. Also, a further advantage is that these low frequency tones havea much better sensitivity compared to higher channel frequencies beingdeployed as a channel frequency key. The tunable filter employed at thereceiver to identify the incoming signal channel grid can be tuned tomatch that grid based upon one, two or more or all the detected tones,tagging signal channels present in the channel grid and then moving thefilter spectrum of the broadband tunable grid filter to match thedetected grid bandwidth. Since any fixed periodic or aperiodic channelspacing will be the fixed across the channel grid, tone keying to thechannel grid using at least one channel tone key can be achieved.

It should be recognized and understood relative to the embodiments ofFIGS. 10-12 that there are two approaches in the deployment of afloating wavelength grid operation in an optical transmission network atthe transmitter. In the first approach, the signal wavelengths arethermally floating because of the lack of temperature control at thePIC, i.e., the signals wavelengths may freely shift with temperaturechanges at the TxPIC while maintaining the spectral spacing betweenadjacent signal wavelengths as fixed value. In this case, the ambienttemperatures of the active elements on the TxPIC may be temperatureadjusted in order that wavelength comb 157 of the transmitted WDM signalcan be more easily detected at the receiver side. This temperatureadjustment may be accomplished with feedback from the receiver to thetransmitter to enhance the rate of achieving grid detection by thereceiver of the thermally floating signal comb 157. In the secondapproach, the wavelength comb 157 at the transmitter may be set at amaximum temperature, T₁, for example, within the designed temperatureoperating range of the TxPIC via operation of the active element heaterson the TxPIC. After the temperature T₁ is reached, the operatingparameters of the laser sources, modulators and PCEs or PDs may be setto be optimized at this temperature. Such parameters are current bias,chirp, modulation extinction ratio and voltage swing limits. Any shiftor drifting of wavelength comb 157 can be continuously adjusted by theTxPIC PLC. In this manner, the receiver can quickly lock onto thefloating wavelength grid of the received WDM signal knowing the setmaximum operating temperature, T₁, communicated from the transmitter tothe receiver as an OSC signal or as part of the WDM signal header of thetransmitted WDM signal.

It is also within the scope of this invention to have more than oneTxPIC at the transmitter so that at least two floating signal wavelengthgrids with fixed channel spacings are present where one wavelength gridis longer than the other wavelength and the two grids do not overlap inwavelength spectrum. In this case, the two TxPICs are operated andmaintained at different temperature levels within the high temperaturerange, such as between room temperature and about 70° C. or more, sothat the grid bandwidths stay within prescribed and separated grid bandsvia transmitter control so that neither grid will walk into the othergrid due, for example, to ambient temperature changes within apredetermined temperature range. At least two demultiplexers at thereceiver, usually preceded by a band deinterleaver, then can lockindependently on the separate grids and demultiplex the incomingmultiplexed channel signals. In this embodiment, the transmitter cantransmit either in the signal frame overhead or via an OSC signalchannel the prescribed boundary conditions of the different signal bandsand their thermal operating range. In this embodiment, it is preferredthat the optical receiver has the capability of receiving many moredifferent wavelength channels than are actually transmitted so that thereceiver is intelligently competent to detect multiple or severaldifferent but spatially separated floating signal channel gridstransmitted by one or more different optical transmitters in the opticaltransmission network.

Also, it will clear to those who are skilled in the art that if thebandwidth of the laser source bandwidth is narrow due to a narrowchannel spacing such as, for example, 50 GHz spacing between channels,then multiple, cascaded demultiplexer stages may be necessary at theoptical receiver to enable good lock-on to the floating signal gridtransmitted from the optical transmitter. A specific example is twocascaded AWG demultiplexer system at the optical receiver respectivelyperforming a filter function, i.e., two different filter functions areperformed. The first filter function is a keying, i.e., the AWGwavelength grid is brought into alignment with the floating wavelengthgrid of the incoming multiplexed channel signals. In such a case, thetransmitter may have sent a key as to its current operating temperatureor a reference wavelength upon which the AWG wavelength grid should bekeyed to. The second filter is deployed to insure low crosstalk existsin the signal path. There may be more than one AWG performing thissecond function with each AWG receiving an output from the first AWG. Inanother embodiment, the cascaded filters could comprise a firstbroadband demultiplexer to discover and tune to the floating but fixedchannel spacing grid of the incoming multiplexed channel signals and asecond narrow band demultiplexer to demultiplex the channel signals as aplurality of channel signals for conversion from the optical domain intothe electrical domain. A further embodiment is for the first filterfunction to be comprised of two gratings with taps, one grating at thelong-wavelength end of the expected spectrum of the incoming signalwavelength grid and the other grating at the short-wavelength end of theexpected spectrum of the incoming signal wavelength grid. When asimultaneous discovery of both the lowest and highest potentialfrequencies of the grid has been achieved, the entire comb of theincoming signal grid has been detected and is locked. The grid of thesecond AWG demultiplexer can be matched to the incoming signal gridbased upon the grid lock-on achieved by the first AWG multiplexer forpurposes of demultiplexing the multiplexed channel signals.

It should be understood with respect to the foregoing described as wellas the embodiments of FIGS. 10-12 that to lock onto an incoming signalchannel grid would preferably include, particularly at startup, an OSCsignal back to the transmitter that a signal channel grid has beensensed or auto-discovered and a lock-on condition has been achieved forproper handshaking methodology.

It should also be recognized relative to any of the foregoing describedembodiments that the transmitter can, at startup, transit an initial OSCsignal or in a dummy signal frame overhead as to what is the designatedstartup temperature to be expected by the receiver. For example, insteadof the receiver starting at the bottom of the acceptable operatingtemperature range, such as, for example, beginning at 40° C., thedesignated startup temperature can be transmitted by the transmitter tobe higher in the temperature range, such as at 50° C. The receiver canthen initially lock the demultiplexer to this temperature or atemperature corresponding to a lookup table suitable for the bandwidthof the demultiplexer which may be different from the bandwidth of theincoming channel grid. After this initial startup procedure, thereceiver multiplexer can continue to detect changes or shifts in theincoming channel grid due to changes in ambient temperature at thetransmitter, which changes can be continuously transmitted from theoptical transmitter or detected via auto-discovery at the opticalreceiver. In either case, if the receiver loses its locked state on theincoming signal channel grid, it can inform the transmitter via an OSCsignal that an unlocked state has occurred and the startup process needsto be reinitiated followed by retransmission of the missed channelsignals.

It will be realized by those skilled in the art that in the forgoingnetwork transmission embodiments of this invention that exemplifythermally actuated AWGs at the transmitter and receiver couldalternatively be electro-optically tuned type rather than of thethermally tuned type. An example of an electro-optically tuned type isdisclosed in U.S. published application No. 2002/0172463, published onNov. 21, 2002 and incorporated herein by its reference. In this example,the lengths of the grating arms of an AWG may be independently varied byapplication of an electric field across each of the several arms to tunethe AWG to match the wavelength grid of a modulated source array ormultiplexed channel signal.

It should be further realized that in connection with the precedingembodiments, in some cases, an off-chip laser source wavelengthstabilization and feedback system of peak wavelengths of the individualsignal channels present on the TxPIC need not be employed but rather afrequency detection system is provided that detects laser sourcewavelengths and/or power, and readjusts and thereafter maintains thewavelength channel spectral spacing between adjacent laser sources onthe TxPIC and/or provide pre-emphasis across the signal outputs of themodulated sources as previously indicated. A preferred way of detectinglaser source wavelength operation to achieve this goal is to utilize anarrow band electrical filter in the feedback system which can, forexample, detect an interference pattern from a pair of fast-responsephotodetectors monitoring one or two laser source outputs. In this case,it would be preferred that these photodetectors be integrated on theTxPIC. This scheme would replace the present larger external etalongenerally employed for laser source wavelength detection. Examples ofsuch dual wavelength, integrated detectors that may be deployed on aTxPIC chip are disclosed in FIGS. 13-19 and 25-27, which are explainedbelow. Other embodiments with multiple on-chip wavelength photodetectorsat the output of the on-chip combiner, e.g., an AWG, are disclosed inFIGS. 20-21, 23 and 24. The most sensitive wavelength monitoring deviceintegrated on the chip would be an AWG multiplexer and can be used, likethe other detector schemes in FIGS. 13-27 for wavelength control insteadof deployment of a conventional external etalon as well known in theart. FIG. 22 operates with one or more optical ring resonators incombination with a single photodetector for each modulated source.However, it should be realized that it is within the scope of thisinvention to utilize offset etalons that are temperature independent fortracking and discerning laser source operating wavelengths. Also, it iswithin the scope of this invention to have etalons that are temperaturematched to associated laser sources to track the laser source emissionwavelengths. In this regard, a preferred embodiment is to integrate suchetalons on the TxPIC chip, e.g., InP-based, integrated etalons.

Before explaining the various embodiments of integrated wavelengthdetectors of FIGS. 13-27, it should be importantly noted that theseintegrated detectors need not be only deployed in PICs that arecoolerless operated via heaters but also may be employed in conjunctionwith PICs that are temperature controlled with coolers, such as withTECs. In other words, is should be clearly understood that the disclosedintegrated detectors herein can be employed in any temperaturecontrolled (cooler or heated) or uncontrolled (floating) environment.

Each of the detector representations of FIGS. 13-19 are integrated dualphotodetectors on the TxPIC monitoring the rear output from the lasersource. In FIG. 13, there is shown one PIC signal channel 170 out of Nchannels with a front PIN photodiode 171, EAM 172 and laser diode source(LD) 173. At the rear facet of laser source 173 is a Y-branch waveguide174 with one end 174A integrated to the back facet of the laser source173 and at the ends of the Y-branched waveguide arms 174B and 174C is aphotodetector 175A and 175B, such as PIN or APD photodiode. First ordergratings 176A and 176B are also in each of the arms 174B and 174C ofY-branch waveguide 174. The center wavelengths of the respectivegratings 176 are offset from opposite sides the target emissionwavelength of laser source LD 173 such that the zero crossing theGaussian outputs of photodetectors 175A and 175B is at the targetwavelength. These two gratings can be made to be sufficiently weak infiltering strength to minimize any detrimental reflective feedback tolaser source 173. Optionally, a phase shift may be deployed in one ofthe Y-branch arms 174B and 174C.

The integrated channel 170A in the FIG. 14 embodiment is the same as theFIG. 13 embodiment except that the two are gratings 176A and 176B areset with center wavelengths at the target emission wavelength of lasersource 173 and ½ phase shift region 177 is formed in one of the Y-brancharms 174C. As long as the target emission wavelength is not atresonance, the photocurrent detected by arm photodetectors 175A and 175B will be different. At resonance, i.e., at the target emissionwavelength, there will be strong light scattering but the response ofthe two photodetectors 175A and 175B will be at a minimum or close tozero due to the grating light scattering at the target wavelength. Witha single target emission wavelength involved in this detection scheme, aphase shift at 177 is required. The backward scattered light towardlaser source 173 will destructively interfere at Y-branch junction 174Athereby suppressing or otherwise eliminating possible interferingfrequency feedback into laser source 173.

In the FIG. 15 embodiment, channel 170B does not include gratings 176 inthe previous two embodiments or a phase shifter 177. Rather, an absorber178 is positioned in one of the arms 174B which provides a complexrefractive index change that provides an photodetector power and phasedifference relative to the light detected by the respectivephotodetectors 175A and 175B.

FIG. 16 is a side elevation of an integrated photodetector pair relativeto one signal channel 180 on a PIC where the channel waveguide portionshown is coupled to the rear fact of a laser source (not shown) viasingle waveguide 181. The concept here includes a high order grating 182having a center wavelength at the target emission wavelength for thelaser source and is deployed in waveguide 181 to eliminate the backwardreflection in the waveguide as well as upwardly scatter the rearwardpropagating light principally to the first of two, out-of-waveguidephotodetectors, PD1 at 183. Photodetector PD2 at 184 will receivecomparatively less light for detection. Since detector 184 is furtheraway from the laser source than the other photodetector 183, the ratioof the absorbed photocurrents of these different photodetectors will bedifferent. At the moment that the target emission wavelength is achievedin the laser source, such as due to its temperature or current biaschange, the ratio of the detected light between photodetectors 183 and184 will go to a minimum because the scattered light will render theamount of light absorbed by either photodetector more equal. As anotherembodiment relative to FIG. 16, the grating in FIG. 16 can be replacewith blazed or angled grating at the target emission wavelength toachieve off-axis reflection and also eliminate backward gratingin-waveguide reflection to end 185 of waveguide 181.

The embodiment of FIG. 17, which is a plan view, operates insubstantially the same way as the embodiment of FIG. 16 except that inchannel 180A one photodetector, PD1, at 186 is outside the cavity ofwaveguide 187, which is coupled to the laser source (not shown), whereasthe other photodetector, PD2, at 188 is in and positioned at the end ofwaveguide 187. Thus, the ratio of photocurrent from photodetectors 186and 188 can be accomplished with one waveguide 187 coupled to the lasersource. The grating 182 is set at the target emission wavelength for thelaser source so that the amount of light received by PD1 at 186 will beminimal at non-resonant condition. When resonance is achieved, such asdue to operating temperature or current bias changes made to the lasersource, a large amount of light will be scattered by grating 182 to bothphotodetectors 186 and 188 so that the ratio of the detected lightbetween these photodetectors will go to a minimum because the scatteredlight will render the amount of light absorbed by either photodetectorto be more equal. Note that grating 182 can be adjusted to scatter morelight to PD2 at 188 rather than to PD1 at 186. Another alternative tothis embodiment is to fabricate photodetectors 186 and 188 to havedifferent lengths, and therefore different absorption lengths, andre-position both photodetectors 186 and 188 to receive more equalamounts of light from the laser source. The ratio of absorption of thephotodetectors will be different so that that when resonance isachieved, the ratio of the detected light between photodetectors 186 and188 will go to a minimum because the scattered light with render theamount of light absorbed by either photodetector to be more equal.

The integrated photodetector arrangement in FIG. 18 for signal channel190 comprises a Mach-Zehnder interferometer (MZI) 192 where arms 193 and194 between couplers 195A and 195B of the device are different lengths(arm 193>arm 194) and functions as a asymmetric MZ homodyne. The phaseshift or other outputs from coupler 195A of MZI 192 are detected byintegrated photodetectors 196 and 197. This detector is highly sensitiveto wavelength changes at the laser source (not shown) and has narrowrange of operation. In one embodiment, this detector scheme may bedeployed for fine tuning the emission wavelength of the laser sourcewhile a coarse adjustment can be handled by another detector arrangementsuch as the vernier based detector arrangement of FIG. 20.

The channel 190A in the embodiment of FIG. 19 is a variant of theembodiment of FIG. 18 except that there is only one coupling region 198.This co-directional coupling region 198 is long in length and designedsuch that there is cross coupling only at certain wavelengths when thelaser source is operating at the target emission wavelength so that thepair of photodetectors 196 and 197 will be at their zero crossing point.The detected outputs can be provided to a differential amplifier wherethe output will null or zero when the detected signals are at a zerocrossover, indicating that they are identical and a lock condition hasbeen achieved. In another embodiment of FIG. 19, the application of adither signal to the laser source can be used so that when a null (peak)is reached, it is difficult to determined if the across over of thesignals may have drifted. By laser source dithering, this undetectedcondition position can be eliminated.

The embodiment shown in FIG. 20 comprises the use of a PIC combiner 200,indicated here as wavelength selective combiner such as an AWG. Itshould be noted that the combiner in this embodiment as well assubsequent embodiments can also alternatively be a non-wavelengthselective combiner or decombiner, such as the multimode interferencecoupler shown in FIG. 27 or a wavelength selective decombiner. Ratherthan the individual laser sources to determine or detect their emissionwavelengths from a plurality if modulated sources if a TxPIC. As shownin FIG. 20, the center output of the AWG is the zero Brillouin ordermultiplexed WDM signal output 201 from AWG 201. The other zero Brillouinorder outputs 202 on adjacent sides of center output 201 havephotodetectors 204 integrated at their ends. The ratio of power in thedifferent photodetectors 204, relative to one another, is a measure ofwavelength shift of the multiple emission wavelengths that appear in theeach of the respective channel outputs detected by photodetectors 204.While the vernier detector of FIG. 20 can provide for wavelengthauto-discovery for fine emission wavelength tuning, vernier outputs 202can also be placed at some multiple temperature shift, e.g., 140 GHz=10°C. change, to provide for a coarse emission wavelength tuningarrangement.

In the combiner 205 of the embodiment of FIG. 21, the higher Brillouinzones (BZs) 106A and 106B, as known in the art, are exact wavelengthreplica of the zero Brillouin order. A photodetector (PD) 201 isprovided for each of the channels represented by the number of BZoutputs 206A and 206B with a passband offset to either side of thetarget wavelength of the respective signal channels. The detectedsignals from the +1 BZ outputs at 206A and the −1 BZ outputs at 206B canfunction as zero crossing differential detectors.

In the embodiment of FIG. 22, there is shown single channel 210 of aTxPIC or of a single EML where there is integrated on the same chip aring oscillator 211 resonating at a given wavelength dependent on thesize of the ring. Ring oscillator 211 can be designed to be set to thetarget emission wavelength of laser source (LD) 212. Ring oscillator 211is coupled on one side to waveguide 213 between laser source 212 andmodulator (EAM) 214 and on the other side to waveguide 215 that containsphotodetector (PD) 216 which may be placed at either end of waveguide215. The detected signal at PD 216 will be at a maximum when theemission wavelength of laser source 212 is the same as the ringfrequency of ring oscillator 211. There can be, in series, a pluralityof coupled oscillator rings 211 but one such ring 211 per channel shouldbe sufficient.

In the embodiment of FIG. 23, there is combination of the integrateddetector concepts shown in both FIGS. 19 and 20. At the two higher + and− Brillouin zones (BZs) at 222A and 222B of combiner 220, plural ringoscillators 224 together with optically coupled photodetectors 226 arecoupled along Brillouin zone (BZ) waveguides 222A and 222B. The ringoscillators 224 are set to have ring frequencies of respective targetemission wavelengths of the respective on-chip laser sources. Thus,there are N combination ring oscillator/photodetectors for N signalchannels of a TxPIC. Here, in this embodiment, N=8. The Brillouin zone(BZ) outputs on waveguides 22A and 22B have multiple combinedwavelengths of the WDM signal. When any one of the wavelengths in thisoutput is in resonance with a given ring oscillator 224 that isindicative that the detected laser frequency is the same as theoscillator ring frequency. A portion of that ring frequency light willleak from ring oscillator 224 to its corresponding PD 226 which is thenindicative that a laser source emission wavelength has been achieved. Inanother embodiment of FIG. 23 t, a wavelength offset technique can bedeployed for wavelength detection or a zero crossing differentialdetector scheme can be used in this embodiment.

The FIG. 24 embodiment relative to combiner 230 is a further variant ofthe embodiment for the combiner 220 shown in FIG. 23 where a set ofon-chip or integrated Echelle gratings 232 is deployed for dispersingthe BZ output at predetermined angles from the gratings 232 dependingupon the wavelength. The dispersed wavelengths of coupling respectivelaser sources are coupled into a respective on-chip waveguide 234 havinga photodetector 236 at its end to detect the amplitude of receivedlight. In another embodiment, the dispersive properties of the Echellegratings can be tuned such that differential detection can be employedusing two different outputs from the respective BZ detected outputs.

The embodiment in FIG. 25 of a single waveguide channel 240 is similarto the embodiment shown in FIG. 16 where detectors 244 and 246 in seriesalong waveguide 242 coupled to the laser source (not shown) but withoutthe use of any gratings. The ratio of the signal detected byphotodetectors 244 and 246 will vary with the wavelength as absorptionof these detectors varies with wavelength. Since the absorption lengthsof photodetectors 244 and 246 change with temperature, photodetectors244 and 246 will also detect wavelength changes with changes in lasersource temperature so that the effect will be magnified between the twophotodetectors because of a difference in the magnitude of the signaldifference of the photocurrent of photodetectors 244 and 246 which isdependent on ambient temperature. Such a magnifying condition is notpossible in the embodiment of FIG. 16 because there is no magnifiedmagnitude due to the presence of the grating In another embodiment, theembodiment alternative may be the same as the embodiment shown in FIG.15, but without the in-waveguide gratings and including absorber 178 inone of the arms 174B and 174C so that the absorber-containing arm 174Bwill absorb light a little differently since, with the presence ofabsorber 178, the phase effect will be magnified between the twophotodetectors because of a difference in the magnitude of the signaldifference of the photocurrent of photodetectors 244 and 246 which isdependent on ambient temperature. As a result, this detection scheme maybe preferred over some of these previous embodiments because of animproved OSNR.

It should be realized that relative to the integrated photodetectorembodiments of FIG. 25 that additional in-series photodetectors can beincluded in the embodiments to enhance detection sensitivity. Also, inthe immediately above another embodiment, similar to FIG. 15, thisembodiment can be extended to include a power splitter at the end ofeach waveguide 242 and have multiple arms extending from each splitwaveguide end and each such waveguide terminated with a photodetector.In this case, with this increase in the number of photodetectors perchannel, the detection sensitivity will be enhanced.

Reference is now made to FIG. 26 which shows a further embodiment foron-chip, integrated wavelength detection. In this embodiment, as inprevious embodiments, only one signal channel 250 is shown comprisinglaser source 251, modulator 252 and PIN photodetector or power changingelement (PCE) 253. The integrated device comprises two-mode interference(TMI) waveguide 254 which is an asymmetrically excited multimodewaveguide that is asymmetrically coupled at 254A of waveguide 250A oflaser source 251, which coupling offset is shown in exaggerated form inFIG. 26. Waveguide 254 has a single mode Y-branch splitter 255 and theends of the waveguides 254B and 254C from Y-branch splitter 255 eachhave a photodetector, PD1 at 256 and PD2 at 257. This device is alsodisclosed in FIGS. 1 at 20, 22, 24a, 24b and 40 in U.S. Pat. No.6,714,566, which patent is incorporated herein by its reference. Theprincipal of operation is similar to a multimode interference (MMI)coupler in that the laser source rear fact output to the on-chipintegrated Y-branch waveguide 254 is offset relative to its multimodewaveguide input at 254A relative to waveguide 250A, which offset excitesthe two lowest order modes of multimode waveguide 254 which then beat toproduce an interference pattern which is wavelength dependent. When thesecond order mode engages Y-branch 255, the modes behave differently inwaveguide branches 254B and 254C to respective photodetectors 256 and257, where the behavioral difference is an indication of changes oflaser emission wavelength with temperature.

Reference is now made to the embodiment shown in FIG. 27 which is awavelength detector in the form of a multimode interference (MMI)coupler 260. Most designs of such couplers are traditionally designed tooperate at resonance so that they are highly insensitive to wavelength.However, for wavelength detection, the coupler must be designed withenhanced wavelength sensitivity. As seen in FIG. 27, input 262 to MMIcoupler 260 has offset rear outputs 263 and 264, such as from twoadjacent signal channels, and where input 262 to coupler 260 is also offcenter. The output power from both channels can be on center or offcenter, as shown, and the two outputs 263 and 264 of these channels canbe coupled to a respective the photodetectors (not shown) at outputs 263and 264 where the crosspoint at 265 between the two Gaussian outputs ofthe photodetectors is an indication of their spatial frequencyseparation or spectral spacing between adjacent signal channels. In thismanner, the desired spatial frequency can be monitored between twoadjacent channels and the wavelength of one or both channels can bechanged to maintain the desired channel spatial frequency separation.

As previously indicated, the on-PIC AWG is the most sensitive device foran on-chip wavelength detection scheme to replace the conventionalexternal etalon which is currently in wide use for transmitter multiplewavelength detection. In an InP-based AWG, although the centerwavelength tunes at a rate of approximately 16.25 GHz/° C., the spectralchannel spacing remains relatively constant. For example, in thetemperature range between about 20° C. and about 80° C., the PIC AWG andthe corresponding DFB laser sources tune across about 1,000 GHz, but thevariation in separation between the channels remains relative small. Forthe DFB laser sources, the range is between about ±20 GHz and about ±30GHz. However, the on-chip AWG is even more stable with a variation ofonly about ±5 GHz, which is approximately only about 0.5% of the totaltuning range. This temperature stability is reason why the on-chipwavelength sensing embodiments of FIGS. 20 to 24 are believed topossibly be the preferred embodiments.

One of the pending issues for broadening the temperature range ofoperation of TxPICs with a floating wavelength grids is the limitationof operating the on-chip EAMs below room temperature, for example, dueto the large detuning of the EAMs at lower temperatures when thesedevices are designed to accommodate for the red shift of the bandedge ofa bandedge EAM with respect to the emission wavelength of itscorresponding channel DFB at higher operating temperatures. This largedetuning results in a poorer extinction ratio and chip behavior of theEAMs. There are two approaches to mitigate this EAM behavior and theextend the operating range of the TxPIC, which is most comfortablebetween about 20° C. and about 70° C., to lower temperature operation.The first approach is to include on the TxPIC in each signal channel aSOA following the EAM and reduce the amount of laser-modulator detuning.The lower detuning would insure proper operation of the EAM at lowertemperatures and the on-chip SOA would compensate for higher on-stateloss of the EAM, which results because of lower detuning of the EAMswith respect to the DFB laser sources. Also, the SOAs would also beemployed to maintain the required output levels at higher temperatureswhere the bias on the laser sources may be with increasing ambienttemperatures. Also, as previously indicated, an on-chip VOA for eachchannel, following a corresponding SOA, can be deployed with a negativebias for purposes of on-chip pre-emphasis across the modulated sourcearray.

The second approach, which has already been previously mentioned anddiscussed, is to provide a heater associated with each EAM, separatefrom the DFB laser source heater, and employ the EAM heaters to maintaina large laser-modulator detuning. The EAM heaters would be operatedbased upon feedback from a coarse thermal sensor or detector, e.g., athermistor, for monitoring the ambient temperature of the TxPIC chip. Asthe temperature of the TxPIC chip falls, for example, the local EAMheaters compensate for the temperature drop by increasing thetemperature of their corresponding EAM and to maintain their optimizedtemperature and thereby maintain the modulator extinction ratio (ER) andits chirp performance. Unlike the DFB and similar light emittingdevices, the dynamic performance of the EAM is less sensitive to changesin temperature, with the exception of laser-modulator detuning whichchanges to a greater degree with temperature.

While the invention has been described in conjunction with severalspecific embodiments, it is evident to those skilled in the art thatmany further alternatives, modifications and variations will be apparentin light of the foregoing description. An important example of this isthat the floating wavelength grid technique of this invention may alsobe deployed in conventional WDM transmission systems having discretetransmitters as long as the transmitters are capable of having thermallyfloating wavelengths within the same temperature ambient. However, it isrealized that with such discrete transmitter devices, it is more likelyto be difficult to control channel spacing among multiple signalchannels. Thus, the approach to conventional WDM systems becomes moreacceptable where the signal channel thermal ambient environment isrelatively small enough so that isothermic changes occur in asubstantially identical manner to all signal channels at the same time.Such a small environment is of a natural consequence, of course, in aTxPIC chip which may have, for example, 10 to 80 channels on a singlesemiconductor chip. Thus, the invention described herein is intended toembrace all such alternatives, modifications, applications andvariations as may fall within the spirit and scope of the appendedclaims.

What is claimed is:
 1. A photonic integrated circuit (PIC) comprising: aplurality of optical sources, each of the plurality of optical sourcesproviding a corresponding one of a plurality of first optical signalsbased on a first temperature, each of the plurality of first opticalsignals having a respective one of a plurality of first wavelengths, theplurality of first wavelengths forming a signal channel wavelength grid,each of the plurality of first optical signals being combined to form awavelength division multiplexed signal for transmission on an opticallink; and a circuit coupled to the plurality of optical sources,wherein, based on a second temperature, each of the plurality of opticalsources outputs a corresponding one of a plurality of second opticalsignals, the circuit configured to: control at least one of theplurality of optical sources such that each of the plurality of secondoptical signals has a corresponding one of a plurality of secondwavelengths, each of the plurality of second wavelengths being uniformlyshifted relative to a corresponding one of the plurality of firstwavelengths, wherein: each of the plurality of optical sources comprisesa corresponding one of a plurality of lasers, each of the plurality oflasers provides a respective one of a plurality of light signals at arespective one of the plurality of first wavelengths, each of theplurality of lasers has an active region with an active regionwavelength, and the respective one of the plurality of first wavelengthsis positively detuned from a peak of the active region wavelength,positively detuning the respective one of the plurality of firstwavelengths causing a gain of the laser to increase as a temperature ofthe laser increases.
 2. The photonic integrated circuit of claim 1wherein the first and second temperature are within a predeterminedtemperature range.
 3. The photonic integrated circuit of claim 2 whereinthe predetermined temperature range is approximately between asurrounding room temperature and a high temperature below 100° C.
 4. Thephotonic integrated circuit of claim 2 wherein the temperature range isbetween about 20° C. and 85° C.
 5. The photonic integrated circuit (PIC)of claim 1 wherein each of the plurality of optical sources is acorresponding one of a plurality of modulated optical sources, and eachof the plurality of first optical signals is a plurality of firstmodulated optical signals.
 6. The photonic integrated circuit of claim 5wherein each of the plurality of modulated optical sources is coupled toa respective one of a plurality of power charging elements.
 7. Thephotonic integrated circuit of claim 5 wherein each of the plurality ofmodulated sources comprises a corresponding one of a plurality of laserscoupled to a corresponding one of a plurality of modulators.
 8. Thephotonic integrated circuit of claim 7 wherein each of the plurality ofmodulators is an electro-absorption modulator.
 9. The photonicintegrated circuit of claim 1 wherein there is no cooler to maintain thePIC at a given temperature.
 10. The photonic integrated circuit of claim1 wherein the first temperature and the second temperature are higherthan a surrounding room temperature.
 11. The photonic integrated circuitof claim 1 further comprising a heater, wherein the photonic integratedcircuit is heated to, and maintained at, a set operating temperature.12. The photonic integrated circuit of claim 11 wherein the setoperating temperature is above a surrounding room temperature.
 13. Thephotonic integrated circuit of claim 12 wherein the set operatingtemperature is below about 85° C.
 14. A transmitter photonic integratedcircuit comprising: a plurality of modulated sources, each of theplurality of modulated sources configured to output a corresponding oneof a plurality of optical signals, each of the plurality of modulatedoptical signals having a corresponding one of a plurality ofwavelengths, the plurality of wavelengths constituting a laser sourcewavelength grid; an optical combiner having a plurality of inputs and anoutput, each of the plurality of inputs of the optical combinerconfigured to receive a corresponding one of the modulated opticalsignals and combine the corresponding one of the modulated opticalsignals into a wavelength division multiplexed signal provided at theoutput of the optical combiner, the optical combiner having a passbandwavelength grid; a heater configured to maintain the operation of thephotonic integrated circuit at an operating temperature above a roomtemperature; and a circuit coupled to the plurality of modulatedsources, the circuit configured to control the wavelengths of each ofthe plurality of modulated optical signals and maintain the laser sourcewavelength grid in substantial alignment with the passband wavelengthgrid of the optical combiner and for optimizing the operation of themodulators with changes resulting from said grid alignment, wherein:each of the plurality of modulated optical sources comprises acorresponding one of a plurality of lasers, each of the plurality oflasers provides a respective one of a plurality of light signals at arespective one of the plurality of wavelengths, each of the plurality oflasers has an active region with an active region wavelength, and therespective one of the plurality of wavelengths is positively detunedfrom a peak of the active region wavelength, positively detuning therespective one of the plurality of first wavelengths causing a gain ofthe laser to increase as a temperature of the laser increases.
 15. Thetransmitter photonic integrated circuit of claim 14 where the circuitcomprises a programmable logic controller.
 16. A transmitter photonicintegrated circuit comprising: a plurality of modulated sources, each ofthe plurality of modulated sources being configured to output acorresponding one of a plurality of optical signals at a respective oneof a plurality of wavelengths; a plurality of photodetectors, each ofthe photodetectors configured to receive a portion of a respective oneof the plurality of optical signals and provide a corresponding one of aplurality of electrical signals, a heater to heat a corresponding one ofthe plurality of modulated sources to an operating temperature above asurrounding room temperature; and a controller, the controllerconfigured to: receive the plurality of electrical signals and change acorresponding one of the plurality of wavelengths of one or more of theplurality of modulated sources so that a spectral spacing betweenadjacent ones of the plurality of wavelengths is maintained at apredetermined one of a plurality of spectral spacings, wherein, each ofthe plurality of modulated sources comprises a corresponding one of aplurality of lasers, each of the plurality of lasers provides arespective one of a plurality of light signals at a respective one ofthe plurality of wavelengths, each of the plurality of lasers has anactive region with an active region wavelength, and the respective oneof the plurality of wavelengths is positively detuned from a peak of theactive region wavelength, positively detuning the respective one of theplurality of first wavelengths causing a gain of the laser to increaseas a temperature of the laser increases.
 17. The transmitter photonicintegrated circuit of claim 16 wherein the heater is a strip heaterformed on one or more of the plurality of modulated sources.
 18. Thetransmitter photonic integrated circuit of claim 16 wherein each of theplurality of electrical signals represents a respective one of theplurality of wavelengths, the controller configured to sense changingemission wavelength conditions due to changing ambient temperatureconditions of the photonic integrated circuit.
 19. The transmitterphotonic integrated circuit of claim 16 wherein the heater is a firstheater, the photonic integrated circuit further including a secondheater for heating the optical combiner.